TECHNICAL FIELD

The technical field of the invention is related to the observation of a sample, in particular a biological sample, with an imaging device operating in ambient light.

PRIOR ART

The observation of samples, and in particular biological samples, by lensless imaging has seen substantial development over the last ten years. This technique allows a sample to be observed by placing it between a light source and an image sensor, without placing any image-forming lenses between the sample and the image sensor. Thus, the image sensor collects an image of a light wave transmitted by the sample, without conjugation between the image sensor and the sample.

Document WO2008090330 for example describes a device allowing biological particles to be observed by lensless imaging. The biological particles are for example cells. The device allows an interference pattern the morphology of which allows the type of cell to be identified to be associated with each cell. Lensless imaging would thus appear to be a simple and inexpensive alternative to a conventional microscope. In addition, its field of observation is clearly much larger than it is possible for that of a microscope to be.

In the visible domain, lensless imaging has been applied to examine samples containing particles, in particular biological particles or cells, for characterization purposes. Examples may be found in WO2017178723, WO2016151248, or WO2016151249. The use of lensless imaging to count particles is described in WO2018115734 or in WO2015166009, or in WO2018060589.

Documents WO2016189257 or EP 319 9941 described the use of lensless imaging to characterize tissue slides of pathology-slide type.

In the aforementioned documents, the lensless imaging is implemented using wavelengths in the visible. In order to keep the ambient light at bay, the devices are such that the main components (light source, image sensor) and the sample are combined in a chamber that is impermeable to light.

The inventors propose a device that is simple to implement, and that allows the constraint on isolation of the sensor with respect to ambient light to be relaxed. A greater ease-of-use results therefrom.

SUMMARY OF THE INVENTION

A first subject of the invention is a method for observing a sample, the sample being placed between a light source and an image sensor, preferably comprising at least 10000 pixels, the light source emitting an illuminating beam, which propagates to the sample, the illuminating beam being emitted in an illumination spectral band lying above 800 nm, the method comprising the following steps:

a) illuminating the sample with the light source;

b) acquiring an image of the sample with the image sensor;

the image sensor being configured such that it has a detection spectral band that blocks wavelengths in a visible spectral band, lying at least between 400 nm and 750 nm, or at least between 400 nm and 780 nm.

Thus, the image may be acquired in ambient light, the image sensor being exposed to light in the visible spectral band.

According to one embodiment, no image-forming optics are placed between the sample and the image sensor. According to another embodiment, the device comprises an optical system, placed between the sample and the image sensor, the optical system having an object focal plane and an image focal plane. The device is then such that:the object focal plane is offset with respect to a plane in which the sample lies, by a focusing distance, which is preferably comprised between 10 μm and 2 mm;and/or the image focal plane is offset with respect to the detection plane, by a focusing distance, which is preferably comprised between 10 μm and 2 mm.

Whatever the embodiment, the method may comprise any one of the following features, implemented alone or in any technically realizable combination:the detection spectral band is comprised between 800 nm and 1200 nm or between 800 nm and 1000 nm;the illumination spectral band is comprised between 800 nm and 1200 nm or between 800 nm and 1000 nm;the illumination spectral band has a bandwidth narrower than or equal to 50 nm, and preferably narrower than 20 nm;the detection spectral band has a bandwidth narrower than or equal to 50 nm, and preferably narrower than 20 nm;the detection spectral band is defined by a high-pass or band-pass detection filter placed on the image sensor, the detection filter being configured to block wavelengths in the visible spectral band; the detection filter may notably be placed between the image sensor and the sample;the illumination spectral band is defined by an illumination filter, coupled to the light source; the illumination filter may notably be placed between the light source and the sample;in step b), the image sensor is exposed to an exposure light wave; the method may then comprise applying a holographic reconstruction operator to the image acquired in b), so as to obtain an image representative of a complex expression of the exposure light wave. The complex expression may be defined on a reconstruction surface, for example a reconstruction plane, lying facing the image sensor, at a nonzero reconstruction distance from the latter. The reconstruction surface is preferably a plane in which the sample lies. The application of the holographic reconstruction operator may be achieved by implementing an iterative holographic reconstruction algorithm, so as to determine a phase of the exposure light wave in the sample plane or in a detection plane in which the image sensor lies.

A second subject of the invention is a device for observing a sample, comprising:a light source, configured to emit an illuminating beam that propagates toward the sample, in an illumination spectral band;a pixelated image sensor, comprising at least 10000 pixels, and configured to acquire an image in a detection spectral band;a holder, arranged to hold the sample between the light source and the image sensor;
the device being configured such that no image-forming optics are placed between the image sensor and the sample when the sample is held on the holder;
the device being characterized in that:the detection spectral band lies above 800 nm;the detection spectral band blocks wavelengths in a visible spectral band, lying at least between 400 nm and 750 nm, or at least between 400 nm and 780 nm.

According to one embodiment, no image-forming optics are placed between the sample and the image sensor. According to another embodiment, the device comprises an optical system, placed between the sample and the image sensor, the optical system having an object focal plane and an image focal plane. The device is then such that:the object focal plane is offset with respect to a plane in which the sample lies, by a focusing distance, which is preferably comprised between 10 μm and 2 mmand/or the focal plane is offset with respect to the detection plane, by a focusing distance, which is preferably comprised between 10 μm and 2 mm.

Whatever the embodiment, the method may comprise any one of the following features, implemented alone or in any technically realizable combination:the detection spectral band is comprised between 800 nm and 1200 nm or between 800 nm and 1000 nm;the image sensor is coupled to a detection filter, defining the detection spectral band;the illumination spectral band is comprised between 800 nm and 1200 nm or between 800 nm and 1000 nm;the illumination spectral band has a bandwidth narrower than or equal to 50 nm, and preferably narrower than 20 nm;the detection spectral band has a bandwidth narrower than or equal to 50 nm, and preferably narrower than 20 nm;the light source is a source of laser light;the light source is a light-emitting diode;the light source is coupled to an illumination filter, the illumination filter defining the illumination spectral band;the device comprises a processing unit configured to apply a holographic reconstruction operator to the image acquired by the image sensor, so as to obtain a complex image of an exposure light wave to which the image sensor is exposed during the acquisition of the image.

Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention, which are given by way of nonlimiting example, and shown in the figures listed below.

