Camera with through-the-lens lighting

Details Camera with through-the-lens lighting Camera with through-the-lens lighting

2001-04-10

2003-11-25

Gray, David (Department: 2851)

Photography

With exposure objective focusing means, focusing aid, or...

Having accumulation photoarray and time control means

C396S100000, C396S121000, C396S234000, C348S296000, C348S297000, C348S348000

Reexamination Certificate

active

06654556

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to cameras with “through-the-lens” lighting and in particular to cameras with “through-the-lens” lighting and a dual-modulation system for protecting the photosensitive surface from stray light.
BACKGROUND OF THE INVENTION
In cameras, true frontal lighting is achieved when the light source is positioned behind the lens, on the lens's optical axis. To reach the image, the light must pass through the lens. With “through-the-lens” lighting there is no shadowing.
The condition of no shadowing is valuable for general photography. It is also important in optical-ranging applications for distance measuring and for acquiring three-dimensional views. Theoretically, in optical-ranging applications, the distance between the camera and a point on a scene is directly related to the amount of reflected radiation from that point. Clearly shadowing introduces errors to the measurements.
An added advantage of “through-the-lens” lighting is that by proper arrangement, it is possible to illuminate substantially only the field of view of the camera. Thus, light losses are be minimized.
Naturally, positioning the light source behind the lens, on the optical axis, would block reflected radiation from the scene; a shadow of the light source would be formed on the photosensitive surface. However, the light source can be located behind the lens, sufficiently offset from the optical axis so as not to block the path of reflected radiation from the scene. The light is boresighted with the optical axis using a beamsplitter or a mirror.
Cameras with “through-the-lens” lighting, using a coherent, collimated, internal light source, are known. U.S. Pat. No. 5,200,793, which is incorporated herein by reference, describes an optical-ranging camera with “through-the-lens” lighting wherein the light passes through an expander/collimator, collimating the beam and modifying its diameter. It is then boresighted with the optical axis using a beamsplitter and directed through a objective lens; the desired field of view is illuminated. A light modulator on the optical axis modulates both outgoing light and reflected radiation from the scene, according to a wave function, generally a sinusoidal wave function. Generally, the light source is laser light. A band-pass filter, adjacent to the photosensitive surface, protects the photosensitive surface from ambient light. However, the band-pass filter does not protect the photosensitive surface from stray light originating from the internal light source (and having essentially the same frequency as reflected radiation from the scene).
SUMMARY OF THE INVENTION
It is an aspect of some preferred embodiments of the present invention that a camera be constructed with “through-the-lens” lighting, wherein a light source is located behind the lens. Preferably, the light is boresighted with the optical axis of the camera, using a beamsplitter or a mirror.
In some preferred embodiments of the present invention, a source of incoherent light is used. Alternatively, a source of coherent light is used.
In some preferred embodiments of the present invention, the light is collimated and the beam diameter modified such that the light that emerges through the lens illuminates substantially only the field of view of the camera.
In preferred embodiments of this aspect, FOV, the field of view of the camera is substantially equal to FOI, the field of illumination. As a result, shadowing is eliminated, illumination of the scene is homogenous and the sense of depth is improved, all these leading to optimal illumination.
Alternatively or additionally, an extended light source is preferably used, wherein the light is not collimated. Preferably the light source has an optical extent substantially the same as that of the photosensitive surface. This means that the effective angle illuminated by the light source as it leaves the camera (FOI) is the same as the effective angle viewed by photosensitive surface (FOV).
An aspect of some preferred embodiments of the present invention is that a camera with “through-the-lens” lighting include a variable-aperture iris, adjacent to the light source, in order to reduce the beam diameter, when desired, such that only a portion of the field of view is illuminated. In preferred embodiments of this aspect, partial illumination of the field of view is made possible. However, generally, the best mode operation is when FOV is substantially the same as FOI.
An aspect of some preferred embodiments of the present invention is that a camera with “through-the-lens” lighting include a dual modulation system comprising a fast light modulator positioned between the beamsplitter (or mirror) and the photosensitive surface and a fast light modulator positioned between the beamsplitter (or mirror) and the light source. A controller, or some appropriate circuitry, controls the two modulators and the light-source. Alternatively, the photosensitive surface itself is switched between an on and off so that it does or does not react to the light incident on it.
In some preferred embodiments of this aspect, a method of dual modulation is carried out which substantially blocks light from reaching the photosensitive surface (or turns the surface off) whenever the light-source is on, yet insures that all the desired radiation reflected from the scene is collected.
Alternatively or additionally, a method of dual modulation is carried out as an effective gain control, to equalize, to some extent, the amount of light reflected from the main object and the amount of light reflected from the background. This serves to increase the signal to noise ratio of a resultant image while avoiding overloading of those portions of the photosensitive surface that image bright parts of the image.
An aspect of some preferred embodiments of the present invention is that a camera with “through-the-lens” lighting be constructed in the CMOS technology, with only one fast light modulator, positioned between the beamsplitter (or mirror) and the light source. Modulation of the photosensitive area is carried out at the level of the pixels. A controller, or some appropriate circuitry, controls the modulation of the light-source and the modulation of the pixels.
In preferred embodiments of this aspect, a program of dual modulation is carried out which substantially blocks light from activating the photosensitive surface whenever the light-source is on. Since gating of the photosensitive area is carried out at the level of the pixels, it is possible to control the amount of radiation reflected from the scene that is collected on each pixel. In this manner, effective gain control, equalizing the amount of light reflected from the main object and the amount of light reflected from the background is better achieved.
There is thus provided, in accordance with a preferred embodiment of the invention, a camera with through-the-lens lighting, comprising:
a light source that produces incoherent light;
a photosensitive surface;
focusing optics, having an optical axis, that forms an image of a scene, within a field of view, on the photosensitive surface,
wherein the light is boresighted with the optical axis and directed toward the scene, illuminating substantially only the field of view of the camera.
Preferably, the focusing optics comprises telecentric optics which collimates reflected radiation from the scene.
Preferably, the focusing optics includes a collimating lens which collimates the light.
In a preferred embodiment of the invention, the camera is an optical ranging camera and comprising a single modulator which modulates both the collimated, incoherent light and reflected radiation from the scene, wherein only a desired, gated portion of the reflected radiation from the scene is collected and wherein intensity values of the image are indicative of distances of objects from the camera.
Preferably, the light source is a modulated light source and the light reaching the photosensitive surface is modulated by a modulator.
Preferably, the light source is modulated and

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