Lasers provide an effective source of coherent energy for illumination. Lasers can provide visible light, such as seen in the use of laser pointers and laser light shows, or infrared or other frequency energy not visible to the human eye. Lasers provide monochromatic light which is particularly useful in certain settings. Because the laser provides a cost effective and source of coherent light, it is particularly well suited to certain applications. Examples include as a light source for projectors such as DLP projectors, as a pulsed light source for time of flight (TOF) measurements in 3D imaging sensors, and for laser based illumination systems. The laser provides a monochromatic light that is also spatially coherent, which makes it particularly useful for laser pointers, for example.
In some applications highly uniform illumination is needed. For example, uniform illumination is required in testing TOF image sensors. While lasers provide a cost effective source of coherent light, the use of lasers for uniform illumination has been thought to be impractical or impossible due to the laser speckle effect.
FIG. 1 illustrates an example of laser speckle that is observed using a prior known laser system. The image 10 in FIG. 1 was obtained by directing a camera at a wall that was illuminated by a laser beam after reflection from a plastic surface (a mobile phone). (Note that the image 10 in FIG. 1 is a drawing in black and white used for the purpose of this patent application, the shading is used to represent the color red that is visible in the original color image.) The speckle pattern is clearly seen. The image illustrated in FIG. 1 is shown in color and described in an article in Wikipedia that is located at the internet world wide web uniform resource locator address http://en.wikipedia.org/wiki/Speckle_pattern.
FIG. 2 illustrates a second example of laser speckle in an image 20. (Note that FIG. 2 is also a black and white illustration of a color image, black and white is used for the purposes of filing this patent application. The shading in FIG. 2 represents the original color image, which was red to purple). In FIG. 2, a laser was directed at a CCD image sensor without a lens. The laser beam from a red laser pointer was directed through a known prior diffuser, a holographic diffuser, and then onto the CCD image sensor. The pattern observed by the CCD image sensor is clearly non-uniform. For an image sensor test application, a uniform illumination is required so that weak or bad pixels can be identified by comparison of charge that is stored in the pixel charge storage element to some expected value. However if the light source used exhibits non-uniform areas, as in this prior known solution shown in FIG. 2, then it is not useful in applications such as testing for image sensors.
Another application of interest is in characterizing image sensors such as are used for time of flight (“TOF”) measurements used, for example, in 3D imaging systems. TOF measurements are used to determine phase differences in a light (which may be visible, infrared or other spectrum) that is directed towards an object and reflected light is then captured at a depth sensor. The phase difference of the reflected light varies with the distance of the object from the sensor and the illumination source, so by measuring the time of flight for different areas, a distance correlation can be determined, providing depth information (depth being greater for objects that are farther from the image sensor or camera). A pulsed light source is often used for TOF measurements. The manufacture of the TOF image sensors requires a uniform illumination source to test the sensors. Laser devices are easily used in such a pulsed light application; however, the illumination must be uniform to be effective for TOF applications.
Prior known solutions for reducing laser speckle require mechanically moving diffusers, such as rotating or vibrating devices, or require electrically active diffusers, placed in the laser beam path. Some prior known solutions use specialized materials that can change the polarization, phase, or direction of the incoming laser light at a fixed frequency. These techniques use active devices that require electronic circuitry to provide a required signal. These solutions require power and can add undesirable mechanical or electrical noise sources to the system. An example prior solution is described in U.S. Pat. No. 6,191,887 B1, entitled “Laser Illumination with Speckle Reduction,” issued to Micahloski et al. on Feb. 20, 2001. Micahloski involves directing laser pulses into a plurality of beam splitters and delay lines to create a plurality of different speckle patterns that are then combined together at a sensor. When the speckle patterns are combined with differing delay paths, the average of the speckle patterns taken together results in a more or less uniform pattern at a sensor. This solution is complex and requires beam splitters, “pulselet” delay lines and temporal separation and spatial aberration of the “pulselets” to achieve the more uniform illumination.
Improvements in laser speckle reduction are therefore needed to address the deficiencies and the disadvantages of the known prior approaches. Solutions are needed that provide uniform illumination from a coherent light source without laser speckle or with substantially reduced laser speckle effects. Solutions are needed that are low in cost, that are robust, and that are easy to implement and use.