Light Leakage

© 20-JUN-2006, Benson Way, Brandeis University

Contents

Apparatus
The Gradient
The Solution
Conclusion

Apparatus

The Proximity Camera is a device that views a Proximity Mask for analysis, alignment, and calibration. The camera itself consists of an IR filter, a lens glued to a lens holder, and an A2047 Proximity Camera Head. Light from the Proximity Mask passes through the filter and is focused by the lens onto a CCD.


Figure: A Proximity Camera viewing a mask.

The Gradient

Upon analysis of one of the cameras (the EOS1_LS_CH_B), we discovered a gradient in the camera image. That is, when viewing a mask, the image appears brighter at the bottom. This effect was also visable in the EOS3_SS_CH_B, but not the EOL1_SS_CH_CH, a similar camera.


Figure: An image from the EOS3_SS_CH_B that shows the gradient.

The Solution

The lens holder has a small hole which the image is supposed to pass through. However, the lens holder exposesthe bottom third of the lens, allowing light to pass through. Believing this to be the cause of the problem, we covered the bottom of the lens using the same sticker used to cover the IR filter caps. As a result, the gradient disappeared.


Figure: Left: A typical lens-holder, with the bottom part of the lens exposed. Right: A lens-holder with the bottom portion of the lens covered.

Figure: Left: The image using the typical lens-holder with a gradient from before. Right: The image with the bottom portion of the lens covered. The gradient has disappeared.

Conclusion

For certain cameras, a gradient is seen on the image. The effect only appears on certain cameras, and it is unclear as to which cameras might have this effect, though we believe that the gradient is more likely to appear for cameras with a small M_PIN distance. The cause of the gradient problem is the exposed bottom portion of the lens. We recommend that if this effect shows up for a camera, the exposed portion of the lens should be covered in order to remove the gradient, improving image quality. However, the method used here to cover the lens may not be a suitable long term solution.