WO2018177565A1

WO2018177565A1 - Device for speckle-free imaging under laser illumination

Info

Publication number: WO2018177565A1
Application number: PCT/EP2017/063565
Authority: WO
Inventors: Uldis Rubins; Edgars KVIESIS-KIPGE; Janis Spigulis
Original assignee: Institute of Solid State Physics University of Latvia
Current assignee: Institute of Solid State Physics University of Latvia
Priority date: 2017-03-28
Filing date: 2017-06-02
Publication date: 2018-10-04
Anticipated expiration: 2019-09-28
Also published as: LV15370A; LV15370B

Abstract

The invention relates to imaging of an object's surface that is illuminated by scattered laser light. The goal of the invention is to ensure speckle-free digital images at illumination that is homogenized by means of reflective diffuser of laser beams. To reach the goal, a device comprising digital image sensor (2) having objective lens, laser irradiation unit (3), reflector (4) having a light scattering coating, mechanically vibrating element (6) and generator (7) of the vibrations is proposed. The reflector having a light scattering coating is fixed to the mechanically vibrating element; the vibration frequency f is selected to satisfy the conditionf t>5,wheretis an exposure time of the digital imaging. The moving part of an acoustic loudspeaker may be used as the mechanically vibrating element. The reflector having a light scattering coating may be ring-shaped, so that the image sensor with objective lens is located inside the reflecting-scattering ring.

Description

Device for speckle-free imaging under laser illumination

Technical field

[001] The present invention relates to imaging technologies, in particular - to speckle-free imaging of object's surface illuminated by scattered light of a number of lasers.

Background Art

[002] Scattered laser illumination has been used in many technical fields, such as monochrome spectral imaging (WO 2013135311 Al, 2012) which can be used for calculation of skin or other objects' absorbing chromophore distribution maps (D.Jakovels and J.Spigulis, J. Biophoton., 3(3), 125-129, 2010; ./. Spigulis et al., Proc. SPIE, Vol. 7557, 75570M, 2010; J.Spigulis et al., Proc. SPIE, v.8216, 82160L, 2012). The laser beam scattering is required to ensure uniform illumination of object's surface. For this purpose beam diffusers placed in the spotlighting are used, which slightly modifies and "mixes" directions of radiation (J.Spigulis and I.Oshina, J.Biomed.Opt., 20(5), 050503, 2015), or diffusive reflectors of specific design (WO 2017/012675 Al, 2015). Although in aforementioned works uniform laser light intensity distribution have been achieved, however, the resulting image quality is lowered by "grains" or so called laser speckles light spots, formed as a result of a coherent radiation interference (https://en.wikipedia.org/wiki/Speckle_pattem). Formation of speckle is determined by laser radiation^'s spectral width, angle of incidence, polarization and wavelength (http://www.siliconlight.com/wp-content/themes/siliconlight/pdf/speckle-spie.pdf). The methods of productive use of laser speckle are known, such as subcutaneous blood flow imaging (D.Jakovels, I.Saknite and J.Spigulis, Proc. SPIE, 9129, 912931, 2014). However, in many cases laser speckles create negative effects to be avoided or reduced - for instance, their negative effect in laser projectors is reduced by using mutually incoherent (randomized) lasers for projection

(https://www.eng.yale.edu/caolab/papers/np.hotonl2.pdf), lasers with broadened emission spectral lines, change of radiation polarization position or by moving diffusers in the spotlighting (http://www.dyoptyka.com/publications/Dyoptyka-PW-8252-4-updated.pdf). Speckle-free images are required also at back- scattered laser beams illumination, used for instance for mapping of object's surface or sub-surface chromophore distribution with RGB digital image sensors (https://www.osapublishing.o.rg/abstract.cfm?uri=Cancer-2016- JTu3A.46).

[003] The known devices cannot provide speckle-free imaging in back- scattered laser light. Disclosure of the Invention

[004] The goal of the invention is to ensure speckle-free digital imaging in lighting, which is homogenized by using reflective laser beam scatterers.

[005] For the achievement of this goal the device is proposed, comprising a digital image sensor having an objective lens adapted for object imaging, a laser irradiation unit, a reflector having a light scattering coating, a mechanically vibrating element, and an oscillation generator. The reflector having a light scattering coating is fixed to the mechanically vibrating element and the overall oscillation frequency / satisfies the condition t >5, where t is exposure time of the digital imaging. As mechanically vibrating element in the device, acoustic speaker moving part(s) can be used. The reflector having a light scattering coating in the device may be ring-shaped, with a protective barrier to avoid illumination of the image sensor by laser radiation, and positioned so that the image sensor with lens is located inside the said ring.

Brief description of drawings

[006] Fig. 1 shows a block diagram of the proposed device;

In Fig. 2 RGB images of skin captured by the smartphone camera at simultaneous laser (448-532-659 nm) illumination are compared: a - with fixed reflector-diffuser and b - with oscillating reflector-diffuser;

Fig. 3 illustrates a possible spatial solution of the device.

[007] The proposed device (Fig. 1) comprises a digital image sensor 2 having an objective lens adapted for object 1 imaging, a laser irradiation unit 3, a reflector 4 having a light scattering coating 5, a mechanically vibrating element 6, and the oscillation generator 7.

[008] For uniform illumination of object's 1 surface, laser radiation reflected from the reflector 4 and scattered by its coating 5 is used; the radiation is emitted from the laser irradiation unit 3. A number of accordingly aligned lasers and their powering devices are placed in said unit that provide the laser operation and their switching-on and/or switching- off. The laser beams through the light scattering coating 5 are directed to the reflector 4, which is connected to the mechanically vibrating element 6, such as an acoustic speaker's moving surface. Oscillator 7 provides an adjustable oscillation frequency f, which is selected to satisfy the condition tf^' >5, where t - exposure time of the image sensor 2, so providing at least five oscillations of the scattering reflector during a single exposure. Even oscillations of some micron amplitude change the diffused laser radiation interference conditions on the object' s surfacel . As a result, laser speckles are periodically moving along the surface. This leads to speckle' s "smudging" during the image capturing exposure, reducing or completely eliminating graininess of the image and thereby improving its quality.

[009] Dimensions of the optical part of the device can be very small - for instance when using a small- sized smartphone speaker, to which moving part a lightweight aluminium foil coated with a diffusely scattering adhesive tape is fixed. The effectiveness of such designed device is illustrated by Fig. 2, where RGB images of skin captured by the smartphone camera at simultaneous 3 laser (448-532-659 nm) illumination are compared: a - with fixed reflector-diffuser and b - with oscillating reflector-diffuser, at a frequency / = 80 Hz and exposure time t = 0.1 s (tf^' = 8). It can be seen that the reflector-diffuser oscillations effectively prevent grains caused by laser speckles on the image of skin.

[010] A geometric location of the reflector-diffuser 4,5 relative to the image sensor 2 can be varied, and its shape may vary as well. A ring shape proved to be the optimal, covering the image sensor and thus ensuring the most even illumination of the object meeting the condition that the used lasers symmetrically irradiate the reflector' s-diffuser's surface. Such a spatial solution of the device is illustrated in Fig. 3. The ring oscillations can be provided by two or more mutually synchronized vibrating elements such as loudspeakers placed at the opposite parts of the ring, ensuring that the loudspeakers' moving parts are in mechanical contact with this ring.

Claims

1. A device for speckle-free digital imaging under scattered laser light illumination, comprising a digital image sensor (2) having an objective lens adapted for object (1) imaging, a laser irradiation unit (3), a reflector (4) having a light scattering coating (5), a mechanically vibrating element (6) and an oscillating generator (7), wherein the reflector (4) having a light scattering coating (5) is fixed to the mechanically vibrating element (6) which oscillation frequency / satisfies the condition ft>5, where t is exposure time of the digital imaging.

2. The device according to claim 1, wherein a moving part of an acoustic speaker is used as the mechanically vibrating element (6).

3. The device according to claim 1 or claim 2, wherein the reflector (4) having a light scattering coating (5) is ring-shaped with a protective barrier against laser illumination of the image sensor (2) and with lens which are located inside the illuminating ring.