EP1891416A1

EP1891416A1 - Method and device for optically measuring a sample

Info

Publication number: EP1891416A1
Application number: EP06762055A
Authority: EP
Inventors: Christian Eggeling; Gerald Donnert
Original assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Current assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Priority date: 2005-06-16
Filing date: 2006-06-15
Publication date: 2008-02-27
Also published as: WO2006133944A1; US20080088839A1; US7719679B2; JP2008544235A; DE102005027896B4; DE102005027896A1

Abstract

The inventive method for optically measuring a sample consists in temporarily repeatedly transmitting an electromagnetic signal (2) to the sample in such a way that a substance contained in the sample is transferred from a first electronic state (1) into a second electronic state (3), wherein at least one part of said substance in the second state (3) emits photons which are used for carrying out the optical measurement of the sample, the signal (2) is transmitted to the same sample area at a certain repetition interval and said repetition interval of the signal (2) is adjusted with a substance whose service life of the second state is approximately of 1 ns on a value of at least of 0,1 µs which is optimised with respect to photon yield of the substance.

Description

METHOD AND DEVICE FOR OPTICALLY MEASURING A SAMPLE

The invention relates to a method for optically measuring a sample, the method having the features of the preamble of claim 1 or. Furthermore, the invention relates to a device for optically measuring a sample having the features of the preamble of claim 19.

The electromagnetic signal for transferring the sample from a first to a second state may in particular be light.

STATE OF THE ART

From DE 101 54 699 A1 a method for optically measuring a sample and a device for carrying out this method are known, which are also referred to as STED microscopy. For fluorescence microscopic examination of a sample, a dye in the sample is first brought into an excited energetic state with excitation light. In this optical excitation, the usual limit for the spatial resolution in optical methods of λ / 2n, where λ is the wavelength of the light used and n is the refractive index of the sample. To fall below this limit, the optically excited state of the dye with de-excitation light is again de-energized to a desired measuring point in which the intensity distribution of the depletion light has a zero point; ie with the de-excitation light, the dye is forced to stimulated emission everywhere outside the measurement point. The dimensions of the resulting fluorescent measuring point, ie the spatial resolution of the remaining fluorescence, can be lowered well below the usual optical resolution limit by applying the excitation light outside the desired measuring point to the sample with such a high intensity that the Depletion by stimulated emission saturation is achieved. Thus, the dye in the sample is only in a very narrow range around the zero of the intensity distribution of the de-excitation light in the fluorescent state. In order to make do with a low average intensity of the depletion, the de-excitation light and thus the excitation light are pulsed. Another reason for the use of pulsed light in STED microscopy is to avoid that the intense de-excitation light loads the sample even when the dye is not excited. This light irradiation, with a small repetition distance sufficiently above the fluorescent dye half-life of typically 1 ns and of the order of 10 ns, is many times repeated for the same regions of the sample, even with only a few dye molecules within a measurement point to still get sufficient from the background noise contrasting measurement signal. The pulses of the excitation light and the excitation light incident in the same regions of the sample have a typical repetition rate of 80 MHz.

At the high intensity of the depletion light necessary for saturation, there is a significant likelihood that the dye in the sample will fade, i. H. is changed so that it no longer emits fluorescent light. Thus, the life of the dye, i. H. significantly reduces the number of times fluorescent light can be registered by him. This limits the yield of fluorescent light from a specific sample in which naturally only a limited number of dye molecules are available.

Similar problems with respect to the yield of photons in the optical measurement of a sample also occur in other pulsed light fluorescence microscopic methods, especially when working as in the multi-photon excitation with high light intensities, and in other methods for optically measuring a sample such as Lifetime measurement of fluorophores (life-time imaging).

In order to improve the yield of photons in methods of optically measuring a sample, detectors with which the photons are registered by the sample, have been improved with considerable effort in terms of their response probability. It has been possible to increase the response probability of high-quality detectors from approximately 20% to 40 to 60% over a period of the past 10 to 15 years. The however, the maximum increase in the yield of photons associated with this has been accompanied by an extreme increase in the cost of the detectors.

Despite the progress of the detectors, it is still the finite total photon yield that sets limits for most fluorescence-based measurement techniques. In fluorescence microscopy, the finite number of total fluorescence dye, i. Photons emitted to bleaching are almost always the main problem. Any significant increase in the absolute signal by increasing the number of pre-bleach fluorescence emissions is of fundamental importance to fluorescence microscopy.

US 2002/0027202 A1 discloses a method for optically measuring a sample having the features of the preamble of patent claim 1 and an apparatus for carrying out this method having the features of the preamble of patent claim 19, which are also referred to as confocal two-photon microscopy. Two-photon microscopy utilizes a quadratic dependence of the fluorescence dye transition probability on the intensity distribution of the excitation light to increase resolution, which it exhibits for transitioning the energy of two photons from its ground state to its excited fluorescent state. In order to obtain the highest possible yield of fluorescent light, the excitation light is concentrated on pulses of high intensity. Due to the quadratic dependence of the transition probability of the fluorescent dye on the intensity of the excitation light, these pulses of high intensity result in the same average power of the excitation light to more fluorescent light than a correspondingly larger number of pulses of lower intensity. In addition, US 2002/0027202 A1 considers a negative influence on the fluorescent light output, which can result from a saturation of the excitation of the fluorescent state during each individual pulse, and advises the intensity of the pulses by means of their repetition rate while the power of the excitation light remains the same adjust that this negative influence on the fluorescent light output just does not start. The danger of bleaching the fluorescent dye is not considered. As basically possible with the repetition rate, US 2002/0027202 A1 specifies a range from kHz to GHz, whereby the repetition rate is to be matched to the lifetime of the fluorescent dye, for which, however, no numerical examples are given. It is known from EP 0666 473 B1 to obtain a comparatively high yield of fluorescent light in two-photon microscopy despite the use of excitation light of relatively low power by using very specific fluorescent dyes, for example in the form of lanthanide chelates, which have a long average life of at least 0.1 μs (1 x 10 ^'7 s), are acted upon with this life matched relatively long pulses of the excitation light. The repetition rate of these long pulses corresponding to the stated lifetime is less than 10 MHz (1 x 10 ⁷ Hz).

The lifetime of the fluorescent state of fluorescence dyes commonly used in fluorescence microscopy is of the order of 1 ns, that is less than 10 ns.

OBJECT OF THE INVENTION

The invention has for its object to provide a method for optically measuring a sample having the features of the preamble of claim 1 and an apparatus having the features of the preamble of claim 20, with which a significant increase in the yield of photons without a costly increase in the response probability achieved by deployed detectors.

SOLUTION

The object of the invention is achieved by a method having the features of claim 1 and by a device having the features of claim 19, preferred embodiments of the new method and the new device are defined in the dependent claims. The dependent claim 25 relates to the use of the new device as a fluorescent light microscope.

DESCRIPTION OF THE INVENTION

Surprisingly, it turns out that the yield of photons from the substance reaches an optimum at an optimized repetition distance of the signal, which is significantly greater than the lifetime of the state of the substance from which they expose the photons sending out. It seems that the signal still further states of the substance to be stimulated directly or indirectly, which have a longer life and the sensible way decay before the signal is again directed to the substance in the same area of the sample. It is thus a departure from the previous approach necessary to increase the yield of photons by avoiding dead times in the optical measurement of a sample by a possible short, ie the lifetime of the photon emitting state of the substance approximate repetition distance. Rather, in the present invention, a meaningful amount of dead time between two repetitions of the signal accepted and just by the yield of photons from the substance increases. It goes without saying that it does not make sense to extend the repetition interval as desired, because this is at the expense of the total duration of the optical measurement. The optimized repetition distance is thus a compromise between maximum yield of photons from the substance and reasonable overall duration of the optical measurement.

When the repetition distance is varied within a range of at least 0.1 μs to 2 μs, the largest increase in the photon yield from the substance is detected by increasing the repetition distance.

By varying the repetition distance, the energy of each individual signal can be kept constant. This is not mandatory. Rather, the energy of each individual signal at the optimized repetition distance can be increased significantly over shorter repetition intervals, without there being an increased risk of bleaching the substance with the signal.

The optimization of the repetition distance can be carried out due to various variants of the considered yield of photons from the substance. If the observed photon yield from the substance is the photon yield from the substance obtained with a given number of repetitions of the signal, which includes the photon yield per repetition of the signal, typically an increase in yield will result with each increase of the photon Repetition distance. The criterion for the optimized repetition distance can then be the achievement of a certain percentage of, for example, 90% of a maximum yield. If the observed photon yield from the substance is the yield of photons from the substance during a certain period of optical measurement of the sample, the optimized repetition distance is that at which the maximum yield in the given time is achieved.

The observed yield of photons from the substance may also be that of a given amount of the substance as a whole, i. H. be yielded to photobleaching until it fades. With this total photon yield from the substance, an increase is expected again with each repeat distance increase, and the optimized repetition distance criterion may be equivalent to achieving a certain percentage of, for example, 90% of the maximum total photon yield from the substance.

It is understood that the optimized repetition distance determined according to the invention is advantageously set for the further optical measurement of the sample. In the new device, this may be done automatically, or the optimizer may output an optimized repetition distance or a plurality of repetition intervals optimized according to different criteria, one of which is selected by the user of the device and then by the optimizer or the user at an arbitrary one Adjusting the repetition distance itself is set for the further optical measurement of the sample.

In a special aspect, in the new method, an optimized time repetition distance, with which the electromagnetic signal, z. For example, an optical light signal repeatedly directed to the same regions of the sample to transfer a substance in the sample from a first to a second electronic state, set greater than a lifetime of a third state, in the parasitic manner a significant portion the substance in the sample is converted due to the electromagnetic signal. In particular, the time repetition distance can be set in relation to the life of such a third state, from which the substance with the signal in a fourth state can be transferred, from which it is not for a period which is longer than the duration of the optical measurement the first or the second state returns. The present invention is based on the surprising finding that a significant risk of inactivation of the substance in the sample assumes that the Substance from a third state, in which it passes in a parasitic manner, but from which it returns relatively quickly to the first or second state, with the signal in a fourth state is convertible, from which they out for a long time or at all no longer returns to the first or second state. In such processes, very small probabilities are sufficient to convert a large proportion of the substance in the sample in the fourth state and thus to inactivate persistently with a very frequent redirecting the electromagnetic signal to the same areas of the sample. However, according to the teachings of the present invention, if the repetition distance of the signal is selected to be greater than the lifetime of the third state, no significant portion of the substance will be in the third state the next time the electromagnetic signal is next directed to the sample. Thus, no relevant parts of the substance can be transferred from the third state with the signal in the fourth state and thus inactivated.

The significant proportion of the passes, the substance in the third state, can with 1 x 10 ^"2 down to 1 10 ^ which is transferred during a single incidence of the signal in the third state x, appear to be very small. Due to the typically very However, a large number of repetitions of the electromagnetic signal during the measurement of the sample can also increase such a small proportion of the substance, which is transferred to the third state, the risk of bleaching the substance considerably, for example, if the life of the third state is significantly greater, of course, as the effective life of the first or second state so that the substance accumulates in the third state, bleaching is also enhanced if the probability that the signal transfers the substance from the third to the fourth state is relatively large ,

Of particular relevance to the aspect of the present invention explained herein are those fourth states in which the substance with the electromagnetic signal from the third

State and from which the substance for a period of time longer than 1 min, in particular longer than 10 min and especially longer than 1 h, does not return to the first or second state. A return or non-return of the substance from the fourth state to the first or second state after this time has elapsed is generally no longer relevant to the optical measurement of the sample according to the present invention, since at least substantial portions of the substance are potentially present in the fourth Condition remain that they are virtually no longer available for the optical measurement of the sample during the period of optical measurement.

As far as here from the life of the third state or the period for which the substance from the fourth state does not return to the first or the second state, this means the respective half-lives of the third and fourth states are meant. It is clear to the person skilled in the art that, even with a very long half-life, parts of the substance leave the respective state much later or even earlier, because these processes are based on transition probabilities which do not result in any transitions at fixed times.

Although the measure according to the invention of increasing the repetition distance with which the signal is directed to the same regions of the sample until reaching an optimum appears simple and in fact does not require a great deal of apparatus in the new apparatus, the effects of the invention are surprisingly large. Thus, in methods for optically measuring a sample in which a fluorescent dye in the sample is exposed to high light intensities by each of the repeated signals, the photon yield may well be increased more than tenfold. A concrete example of this is the STED microscopy described in DE 101 54 699 A1, in which the de-excitation light has a particularly high intensity, and fluorescence microscopy with multi-photon excitation, which also requires very high light intensities. Fundamental advantages also arise in other methods for optically measuring a sample, such as, for example, the life-time imaging with pulsed light.

It is important to point out that the electromagnetic signal, such as the light signal, in the context of the definition of the present invention, various components, ie, for example, portions of different wavelengths or even sequential parts may have. With the invention, the repetition distance for the electromagnetic signal is set, which is directed to the optical measurement of at least one measuring point once to the sample.

The optimized repetition distance of the electromagnetic signal in the present invention is at least 0.1 μs. This corresponds to a maximum repetition rate, with which the signal is directed to the same areas of the sample, of 10 MHz. Is preferred it, if the repetition distance of the electromagnetic signal amounts to at least 1 μs. This corresponds to a repetition rate of less than 1 MHz. When determining the repetition rates corresponding to the repetition intervals, the duration of the respective electromagnetic signal must also be taken into account so that in each case a slightly lower value than the reciprocal of the repetition distance results.

As already indicated, in the present invention, it makes no sense to arbitrarily reduce a repetition frequency at which the electromagnetic is directed to any portions of the sample to optically measure them. This would significantly increase the duration of the optical measurement, within which a certain absolute yield of photons is achieved, so that, for example, the signal-to-noise ratio could be severely degraded or stability problems could occur. It is therefore preferable if the repetition frequency of the electromagnetic signal is at least 100 kHz. However, a significantly lower repetition frequency can be tolerated if the sample is optically measured simultaneously in a number n of measuring points, because this increases the photon yield per time by a factor of n. Accordingly, then the repetition frequency of the signal can be reduced to 100 kHz / n. For example, if the sample is exposed to the electromagnetic signal over its entire area and is observed by a camera, it is possible, for example. B. the number of pixels of the camera are equated with n, so that n can be quite large. By reducing the repetition frequency proportionally to 1 / n, however, no appreciable advantages with respect to the maximum yield of photons from the substance in the sample until its bleaching are achieved, at least for larger values of n. These advantages are already at an increase in the repetition distance of the electromagnetic signal on depending on the substance about 0.1 μs to 10 microseconds.

To practice the present invention, a repetition frequency of the optical signal directed to the sample may be tuned down to increase the repetition distance. Such a repetition frequency is currently on the order of 80 MHz in most optical methods in which an optical signal is repeatedly directed to the sample. To carry out the invention, this repetition frequency is lowered to, for example, 500 kHz. If the electromagnetic signal is only directed to at least one spatially limited area of the sample, instead of reducing the repetition frequency of the electromagnetic signal, a sampling rate at which the sample is spatially scanned with the signal can be increased by the time repetition distance enlarge. Both measures, ie the reduction of the repetition frequency and the increase of the scanning speed can be used together to set the repetition distance to a value according to the invention. It is important that the repetition distance, with which the same area of the sample is repeatedly subjected to the electromagnetic signal, complies with the minimum dimension according to the invention. For this purpose, such rapid scanning of the sample with the electromagnetic signal contributes to the fact that the electromagnetic signal is not incident twice in succession on the same regions of the sample, but that the respective areas acted upon by the optical signal do not overlap. In this case, the repetition distance at which the electromagnetic signal is directed to the same area of the sample can become very large without the photon yield per unit time decreasing.

When the sample is scanned with the electromagnetic signal, the photons from the sample are preferably registered with a one- or two-dimensional photosensor array onto which at least a portion of the sample is time invariant mapped. Each spatially limited area of the electromagnetic signal which shifts when the sample is scanned relative to the sample is thus continuously imaged onto other light sensors of the light sensor array, but the same light sensor always registers photons from the same area of the sample. Thus, with the fast scanning of the sample according to the invention no problem is associated with the separation of photons from different regions of the sample. The light sensor array may be a camera, in particular an electronic camera, such as a CCD or CMOS camera.

By sampling the sample with the electromagnetic signal sufficiently fast, the signal may even be continuous, or altogether a continuous signal, which, due to the high scanning speed with which the sample is scanned, is only temporarily directed to the individual regions of the sample , The optical signal can therefore come wholly or partly from a continuous wave laser. The signal increase by very fast scanning at very high scanning speeds is also surprising for the skilled person, because the skilled person would not expect an advantage of a faster Abrastem the sample in and of itself. This misunderstanding is facilitated by the misconception that you have to measure at one point until you have collected enough signal. The fast scanning therefore initially appears to the skilled person only as an unnecessary complication.

The implementation of the new method can be carried out in the field of fluorescence microscopy. Specifically, the signal may include excitation and de-excitation light for excitation and spatially limited re-excitation of the subject in a STED process. The signal may also comprise excitation light for, in particular, multiphoton excitation of a fluorescent dye or, more generally, a fluorophore as a substance in the sample.

In the second state, in which the substance is transferred with the electromagnetic signal and from which it emits photons, it can also be a virtual state (non-intrinsic state) of a molecule in the sample, which is extremely short-lived (<1 fs ) but is involved in a (non-linear) scattering process of the irradiated electromagnetic signal, whereby the scattered light generated by it by (non-linear) scattering is used as the measurement signal. Such nonlinear stray light includes i.a. Higher Harmonics (Second and Third Harmonics) and Coherent Antistokes Raman Scattering (CARS).

The intensity profile of the signal which is repeatedly directed onto the sample at the repetition distance according to the invention may vary. It may consist of both a single and a plurality of individual pulses, not only the already mentioned combinations of pulses of different wavelengths as in STED microscopy are meant. The signal can also have a plurality of individual pulses of the same wavelength, for example in the form of bursts, in which the frequency of the individual pulses has a usual high order of magnitude of 100 MHz or more. The number of individual pulses of the same wavelength in the signal can be in a range of 2 to 100, for example. However, it is important that the signal ends and the repetition interval begins when a considerable proportion of the substance has already been transferred to the above-mentioned third state over the previous signal. When the transition probability to the third state is small, the number of single pulses of the same wavelength in each can single signal also be greater than 100. In any case, the proportion of the substance in the third state has sufficient time to decay during the following repetition interval.

Advantageous developments of the invention will become apparent from the dependent patent claims and the entire description. Further features are the drawings - in particular the illustrated geometries and the relative dimensions of several components to each other and their relative arrangement and operative connection - refer. The combination of features of different embodiments of the invention or features of different claims other than the selected back relationships is also possible and is hereby stimulated. This also applies to those features which are shown in separate drawing figures or are mentioned in their description. These features can also be combined with features of different claims.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention will be further explained and described with reference to the figures based on preferred embodiments.

Fig. 1 shows various electronic energy states of a fluorescent dye.

Fig. 2 shows the scanning of a sample with a single measuring point in the optical

Measuring the sample, and

Fig. 3 shows the scanning of a sample with several measuring points in the optical

Measuring the sample.

Fig. 4 shows the increase in the total yield of photons with pulsed excitation of a common fluorophore by 1-photon absorption at 470 nm and a fixed pulse duration of 100 picoseconds by 7.7 times by reducing the repetition frequency from 40 MHz to 500 kHz. Fig. 5 shows the increase in the total yield of photons at pulsed excitation of the green fluorescent protein GFP by 2-photon absorption at 900 nm and a pulse duration of about 200 femtoseconds by 14.3 times by reducing the repetition frequency of 40 MHz 500 KHz.

DESCRIPTION OF THE FIGURES

The electronic states of a fluorescent dye schematically represented in FIG. 1 comprise a ground state 1 which corresponds to the lowest energy of the fluorescent dye at all and in its singlet state S. By means of an electromagnetic signal 2, the fluorescent dye can be converted into an excited state 3, which likewise belongs to the singlet state of the fluorescent dye and which may be the fluorescent state of interest of the fluorescent dye from which it emits, while emitting recordable fluorescent light 4 in FIG the ground state 1 can return. In addition, there is some likelihood that the fluorescent dye will enter a third state 5, which is the lowest energy state within the triplet state of the fluorescent dye. In Fig. 1 it is indicated that the fluorescent dye reaches state 5 out of state 3 out. But it is also possible that the fluorescent dye passes directly from the state 1 due to the signal 2 in the third state 5. If a lifetime that determines a return of the fluorescent dye from the third state 5 in the ground state 1 or in the excited state 3 is greater than a repetition distance of the signal 2, by a rapid repetition of the signal 2, the state 5 to a pumped to a certain level, which is determined by the exact ratios of the transition probability in the state 5, the life of the state 5 and the repetition distance of the signal 2. Ie. a certain proportion of the molecules of the fluorescent dye of typically a few percent is constantly in the state 5 and thus is not transiently available for the emission of fluorescent light 4. However, these molecules of the fluorescent dye in the state 5 are basically still available and also send back fluorescent light 4 after returning to the state 1 or 3. Therefore, the fluorescent dye in the state 5 need not be considered bleached, ie no longer available for the optical measurement of a sample. However, it is different with proportions of the fluorescent dye that from the third state 5 under excitation by the signal 2 in a fourth state. 6 from which they no longer return to the ground state 1 or the excited state 3 because either the fourth state 6 has a very long life, or from the fourth state 6 out a chemical change, ie a destruction of the fluorescent dye takes place.

The typical lifetime of the fluorescent state 2 of fluorescent dyes is of the order of 1 ns. The lifetime of the condition 5 considered here is typically at least an order of magnitude higher. The half-life at which the fluorescent dye returns from state 6, either via state 5 or otherwise, to one of states 1 and 3, if it returns at all, is several orders of magnitude greater, so that numerically low probabilities that the Fluorescence dye in the state 5 and from the state 5 in the state 6 has a significant influence on the number of times in which the fluorescent dye can emit fluorescent light 4 before it fades, d. H. becomes inactive in some way.

The precipitation of the fluorescent dye, however, because it enters the state 6, is, according to the invention, suppressed by increasing a repetition distance of the signal 2 beyond the lifetime of the third state 5. On the one hand there is no significant occupation of the state 5 at all, and on the other hand there is the danger that significant portions of the fluorescent dye which are in the state 5 are supplied with the signal 2 and thus in the state 6 arrive, eliminated.

Since only the same molecules of the fluorescent dye are important for the consideration of the repetition distance of the signal 2, that is to say that the signal 2 does not impinge on the same regions of a sample with too high a repetition rate, instead of increasing the repetition interval of the signal 2, the signal Reduction of its repetition frequency corresponds to the region of the sample to which the signal 2 is directed to be changed so fast that the signal 2 does not hit its own repetition frequency on the same areas of the sample. FIG. 2 outlines the scanning of a sample 7 with a measuring point 8 which corresponds to the area in which the sample 7 is supplied with the signal 2. If the measuring point 8 is moved over the sample 7 in the indicated direction so fast that the signal 2 no longer strikes the same area of the sample 7 as before for each of its repetitions, the same effect is achieved as if the repetition frequency of the signal 2 itself would be reduced. With sufficiently fast movement tion of the measuring point 8 on the sample 7, the signal 2 may even be a continuous signal and still act in the individual measuring points 8 as a transient signal.

FIG. 3 indicates how, by increasing the number of measuring points 8 with which a sample 7 is scanned, the total yield of photons per time can be increased even if the repetition frequency of the signal 2 for each of the measuring points 8 is reduced. In the case of the illustrated 10 measuring points 8, the repetition frequency of the signal 2 at each measuring point 8 can be reduced to one-tenth, without this being a loss of excitation of the sample 7 by the signal 2 compared to a measuring point 8 in which the repetition frequency is not reduced 1 means according to FIG. At the same time, the yield of photons in each measuring point 8 drops only disproportionately as a result of the described reduction of the repetition frequency of the signal 2, since the relative proportion of the fluorescent dye which changes to state 5 or state 6 according to FIG. 1 is reduced of photons per time at 10 measuring points instead of a measuring point with 10 times the frequency of the signal 2 even larger from the outset. The difference increases more and more with time, because almost no molecules of the fluorescent dye more enter the state 6 and thus fail permanently for the optical measurement of the sample 7.

It is part of the teaching of this invention that a repetition rate characteristic of the sample of the impinging electromagnetic signal provides a maximum total photon yield. This can also be a repetition rate range in which the maximum total yield of photons is wholly or at least substantially achieved. Within this repetition rate range, however, it is preferable to choose the highest possible repetition rate at which not only the total fluorescence yield is high, but also the total measurement time is as short as possible. From the series of measurements for the dye Atto532 (excitation wavelength 470 nm, 1-photon absorption) shown in FIG. 4, it can be seen that a repetition rate of 500 kHz-1 MHz is a good choice. At this repetition rate, compared to a 40 MHz repetition rate typical of the prior art, the total yield of photons from the fluorescence of the dye is increased 6.5 to 7.7 times.

The measurements to determine the maximum yield of photons from the fluorescence of the

Dye Atto532, the results of which are shown in Fig. 4, were carried out on a fixed Atto532 dye layer in a grid block pattern of 32 different locations (4x8). In each block, the power is doubled line by line, so that the darkest spots correspond to the highest laser power. The increase in power and the grid itself are not essential here. It is essential that the recording of the raster blocks was carried out with different laser pulse rates: 40 MHz, 10 MHz, 1 MHz and 500 kHz. The recording time at one point was adjusted according to the repetition rate so that each point was hit by the same total number of pulses. Overall, only the time interval between the pulses is varied, but not the integral radiation exposure. The maximum power was 40 μ W at 40MHz. The time courses show the fluorescence yield at a fixed location of the sample with the same peak power and the same total number of pulses on the dye molecules, measured at the 4 laser pulse repetition rates. The integral over the measuring time shows an increase of the fluorescence yield by a factor of 7.7 between the measurement at 40 MHz and the measurement at 500 kHz. At the same time, the reduction in dye bleaching is evident with the reduction in the repetition rate in the images.

From Fig. 5 it is apparent that in the very important for fluorescence microscopy green fluorescent protein (GFP) at a 2-photon excitation at the wavelength of 900 nm even an increase in the total yield of photons by a factor of 10 -14.3 results by placing the repetition rate of the excitation light pulses instead of the usual 40 MHz in the interval of 500 kHz to 1 MHz.

The measurements, the results of which are shown in Fig. 5, were carried out on a fixed layer of green fluorescent protein (GFP). As with the previous ones

Results according to FIG. 4 were each given the same peak power and corresponding to the

Repeat rate of adjusted recording time measured (maximum power 4 mW at

40MHz). Optimizing the repetition distance of the pulses gives GFP an increase in the photon efficiency by a factor of 14.3 compared to measurements at 40 MHz and 50OkHz. The reduction of the fluorophore bleaching is also directly detectable in this case by the bleaching images. LIST OF REFERENCE NUMBERS

ground state

signal

Status

fluorescent light

Status

sample

measuring point

Claims

A method of optically measuring a sample, wherein an electromagnetic signal is repeatedly directed to the sample repeatedly to transfer a substance in the sample from a first electronic state to a second electronic state, wherein at least a part of the substance is from the second state emits photons used to optically measure the sample, - the signal being directed at the same areas of the sample at a time repetition interval, and - the time repetition distance of the signal being set to a value that is relative to a sample Yield of photons is optimized by the substance, characterized in that - the time repetition distance of the signal is set at a value of at least 0.1 microseconds for a substance in which a lifetime of the second state has an order of magnitude of 1 ns.

2. The method according to claim 1, characterized in that the lifetime of the second state of the substance is greater than 0.1 ns and less than 10 ns, and preferably between 0.5 and 5 ns.

3. The method of claim 1 or 2, characterized in that the optimized at the value of the time repetition distance yield of photons from the substance is the total of a certain amount of the substance obtained photon yield.

4. The method according to any one of claims 1 to 3, characterized in that the value of the time repetition distance of the signal (2) is greater than a lifetime of a third state (5), in the parasitic manner, a significant portion of the substance in the sample (7) is converted on the basis of the signal (2), - wherein the life of the third state is at least an order of magnitude longer than that of the second state, and - wherein the substance from the third state (5) with the signal (2) in a fourth state (6) is transferred, from which it does not return to the first state (1) or the second state (3) for a period of time which is longer than one hundred times the life of the third state (5).

5. The method according to claim 4, characterized in that the substantial part of the substance with a single incidence of the signal (2) at least 1 x 1CT ⁴ and in particular at least 1 x 10 ^{~ 2 of} the substance.

6. The method according to claim 4 or 8, characterized in that the period for which the substance from the fourth state (6) does not return to the first state (1) or the second state (3), more than 1 min, in particular more than 10 minutes and more particularly more than 1 hour or that the substance from the fourth state (6) no longer returns to the first state (1) or the second state (3).

7. The method according to any one of claims 1 to 6, characterized in that the value of the time repetition interval is at least 1 microseconds.

8. The method according to any one of claims 1 to 7, wherein the sample is measured optically in n measurement points simultaneously, characterized in that a repetition frequency of the optical signal, with which it is directed to any portions of the sample during further optical measurement of the sample, at least 100 kHz / n.

9. Method according to one of claims 1 to 8, wherein the signal is directed in each case to at least one spatially limited area of the sample, characterized in that the value of the time repetition distance by means of a scanning speed at which the sample (7) with the at least one spatially limited area is scanned spatially.

10. The method according to claim 9, characterized in that the photons are registered by the sample with a one- or two-dimensional light sensor array on which at least a portion of the sample is time invariant imaged.

11. The method according to claim 9 or 10, characterized in that the signal has at least one continuous component.

12. The method according to any one of claims 1 to 11, characterized in that the signal (2) comprises excitation light and de-excitation light for excitation and the spatially limited depletion of the substance.

13. The method according to any one of claims 1 to 12, characterized in that the signal (2) comprises excitation light for multiphoton excitation of the substance.

14. The method according to any one of claims 1 to 13, characterized in that the signal (2) comprises light for non-linear multiphoton scattering of the substance.

15. The method according to any one of claims 1 to 14, characterized in that the signal in each case has a number of individual pulses of the same wavelength.

16. The method according to claim 15, characterized in that the number of individual pulses of the same wavelength in the signal is in a range of 1 to 100.

17. Apparatus for optically measuring a sample - the apparatus repeatedly repetitively directing an optical signal to the sample to transfer a substance in the sample from a first electronic state to a second electronic state, the device comprising photons derived from at least a part of the substance to be emitted from the second state, used for optically measuring the sample, - wherein the device directs the signal with a time repetition distance on the same areas of the sample, - wherein an adjusting means for setting an optimized time repetition distance of the signal is provided in which the value of the time repetition interval of the signal is adjustable to at least 0.1 μs, and wherein a scanning device is provided for spatially scanning the sample with the signal, characterized in that a scanning speed of the scanning device is established with the adjusting device is ellable to adjust the value of the time repetition distance.

18. The device according to claim 17, characterized in that the value of the time repetition distance of the signal with the adjusting device is adjustable to at least 1 μs.

19. The apparatus of claim 17 or 18, characterized in that an optimizing means for optimizing the value of the time repetition distance of the signal is provided, - varies over the adjusting the repetition distance of the signal and an accompanying change in a yield of photons from the substance registered, and - which determines a value of the time repetition distance optimized with regard to the yield of photons from the substance.

20. The device according to claim 19, characterized in that the registered yield of photons from the substance comprises the total of a certain amount of the substance obtained photon yield.

21. Device according to one of claims 19 and 20, characterized in that the optimization means automatically adjusts the determined optimized value of the time repetition distance for the further optical measurement of the sample via the adjusting device.

22. Device according to one of claims 19 to 21, characterized in that the optimization means outputs the determined optimized value of the repetition distance.

23. Device according to one of claims 17 to 22, characterized in that a recording device for the photons of the sample one or two-dimensional light sensor array, is imaged at least a portion of the sample invariant in time.

24. Device according to one of claims 17 to 23, characterized in that a continuous wave laser for providing the optical signal (2) is provided.

25. Use of a device according to one of claims 17 to 24 as a fluorescent light microscope.