DLP670S: Abnormal phenomenon of diffraction spot energy distribution in DLP670s

Hello Zeb,

Your analysis seems sound.  How far away is the screen from the DMD?  Or stated another way, what is the angle between the orders in the picture?

Also since the energy seems to be almost equally split along the x direction, which of the two brightest dots represents the "middle"?  If you turn off the DMD the brightest order should be the 0th order.

Fizix

Thank you for your reply. At your suggestion, we supplemented the experiment again to compare the diffraction spots of DMD in the two states of Power Off and Solid Field. However, some unusual phenomena were found:

1. In the Power Off state, the micromirror itself does not seem to be in a horizontal state with the base, and there is a certain angular deviation, causing the light spot in the long side direction of the screen to shift.
2. When switching between the two states of Power Off and Solid Field, it can be clearly observed that the brightest light spots have undergone considerable positional shifts in the X direction.

The above phenomenon makes it impossible for me to confirm the specific position of the 0th light spot, but it is certain that when the two states are switched, the tilt angles of the micromirror in both orthogonal directions have changed simultaneously.

The following two pictures show the relevant parameter information of our experiment. Due to limited conditions, there are certain errors in the measured data. We hope it can help you analyze the causes. We are very curious about what the specific posture of the micromirror is in the Power Off state?

Hello Zeb,

This is very interesting.  Please allow me to consult with our optics team on this question.  I will try to answer on Friday.

Fizix

I was able to talk to the Optics team today.  Before we give an answer, I have one more question.  In the Image above of Power Off, which direction does the energy move if you turn all the mirrors off?  To the left or to the right?

Hello Fizix

Thank you for your continuous attention to this issue. I'm not sure if your question is understood in this way correctly: When powered on, if the entire micro-mirror is set to Off, that is, in the case of a completely black image, what is the direction of the light spot's movement at this time?

When powered on, the micromirror is placed in the Off state representing the black pixel. At this time, the energy of the diffraction spot will shift to the right by a considerable distance, which is opposite to the direction of shift when it is in the Power Off state (When the DMD is not connected to the power supply).

Zeb

Hello again Zeb,

You understood the question perfectly.  Thank you for the answer.  In your image of the "Solid Field" I then say that the order to the left out of the two is the zero order line.  It appears that the mirrors have become slightly torqued to the right (toward the "off" position).

When was this unit purchased?  Did it look this way from the very beginning or did this develop slowly?  Do you operate it with the the mirrors in the "off" (black screen) for long periods of time?

Also, in your later pictures what is causing the spots to be long rectangles?

Fizix

Hello Fizix, 

Thank you for your continuous efforts in helping us trace the cause of the problem.

1. This DMD was probably purchased around November or December last year. When we first purchased it, we didn't conduct any tests in this regard, but we believe it should be a common phenomenon.
   • The concern for this phenomenon is due to the fact that when we design projectors using DLP4710 and DLP670s screens, there is always an uneven brightness on the left and right sides of the screen, which is particularly obvious when the lens diaphragm is small. For related questions, you can refer to "DLP670S: Questions about the tilt Angle error of the micro-mirror and the causes of the color stripes on the screen.
   • Considering that the micro-mirror tilts towards the side closer to the long side when it is in the "on" state, we think it is understandable that there is an uneven lighting distribution up and down along the short side of the screen at this time. However, the common unevenness of left and right lighting in the long side direction has been a problem that has troubled us for a long time.
2. We did not keep the mirror in the "off" state (black screen) for a long time. The DMD is mainly used to form phase-shifted black and white stripes.
3. The change in the shape of the light spot in the previous and subsequent pictures is merely due to our adjustment of the laser's focusing, increasing the size of the light spot and forming a long horizontal light spot. This is to make the phenomenon more obvious when taking photos.

Zeb

Hello Zeb,

Thank you for your explanation.  It is much clearer now.

By "small lens diaphragm" I am assuming you mean a small numerical aperture (NA) / large f#.  Small NA systems will make any non-uniformities in tilt and pixel topography very obvious.

I sent you a friend invite, so that we can set up a conference call with the DLP Optical team on this matter.  Could you accept the friend invite?

Fizix

Hello Fizix, 

It's a great pleasure to communicate and exchange with you to solve any doubts. In order to have sufficient time for a friendly discussion, please allow our team to confirm the arrangement for next week first (as we may be on a business trip). After confirmation, we will jointly agree on a communication time with your team.

Zeb