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About dispersion property of DLP3000

Xuanwu Yin
Prodigy 70 points

I use the LightCrafter to develop an application with its DLP3000. The light engine has been removed. I intend to use white light generated by a halogen tungsten lamp to illuminate the DMD chip, and use the DMD as a group of mirrors that can be controlled via programming. However I found that there is dispersion problem. The reflected light seems to be colorful. Is it related to the angle of the incident light? How can this problem can be solved? Is it possible to use white light as the source? Thanks.

  • 0
    Prodigy 70 points

    ****************************MERGED FROM ANOTHER FORUM****************************

    I use the LightCrafter to develop an application with its 0.3" DMD. The light engine has been removed. I intend to use a white light source to illuminate the DMD chip, and use the DMD as a group of mirrors that can be controlled via programming. However I found that there is dispersion problem. The reflected light seems colorful. How can this problem can be solved? Is it possible to use white light as the source? Thanks.

  • 0
    TI__Guru 54175 points

    Hello Xuanwu,

    Welcome to the DLP section of the TI-E2E Community.

    You have correctly identified that the resulting light output shows chromatic dispersion.  This is because the DMD acts as a 2 dimensional grating.

    One possibility is to use a low f /# system (i.e. f /2.4 - f /3 for illumation and projection) that re-images the DMD on the target.  This will have suffiecient distribution of angles to mask the dispersion.

    The only other possibility to solve this issue is to introduce a prism that has the inverse dispersion of the DMD so that he waveleght are recombined to the same direction (i.e. undo the dispersion).

    Fizix

  • 0 in reply to Fizix
    Prodigy 70 points

    Let me describe our application briefly.

    1. light pass the main lens to form an image

    2. a relay lens is put behind the main lens, and converts the light into parallel rays

    3. the parallel rays are modulated and reflected by DMD

    4. another relay lens is used to re-image the modulated light

    The dispersion appears in the 3rd step. Do you mean that using a low f/# relay lens can solve this problem? Can the TIR prism act as the anti-dispersion prism? Thanks.

  • 0 in reply to Xuanwu Yin
    TI__Guru 54175 points

    Hello Xuanwu,

    Can you describe your application a little more.  What is it that you are attempting to accomplish?  It sounds like you are attempting something more than a simple projector.

    Fizix 

  • 0 in reply to Fizix
    Prodigy 70 points

    We are attempting to use the DMD as the spatial light modulator, to accomplish an imaging system. Traditionally, light rays pass through the main lens and image on the focus plane. In our system, since it is hard to insert DMD into the main lens, we use relay lens to turn the image of the main lens into parallen rays, and the second relay lens re-image the rays on the sensor plane. It is equivalent to operate inside the main lens, if the f number of the relay lens and the main lens match. 

    So the problem comes to: 

    In what way we can use DMD to reflect natural (or white) parallel rays into parallel rays, without dispersion.

    Would you give some suggestions? Thanks.

  • 0 in reply to Xuanwu Yin
    TI__Guru 54175 points

    Hello Xuanwu,

    Typically in a projector the DMD surface is imaged at the screen.  The dispersion is only in angle space and as long as you underfill the output pupil with the illumination then all of the rays will reconverge at the image.

    I am not understanding the need to operate in columnated space (i.e. between the two relay lenses), which is equivalent to using the DMD with a very small NA.  This will cause dispersion or any variation in production tilt angle to move light out of the projection bundle.

    Are you able to image the illumination bundle onto the DMD and then re-image the DMD at the desired target?

    Fizix