QuickCam Team Forums

I have an application for a webcam that requires measuring the light output of an array of variable intensity LEDs in a dark room. The array is typically 7x7, but may be up to 15x15. I'm currently using a Logitech QuickCam Deluxe for Portables.

I have software that extracts "spot" information from a video frame (typically 320x240 pixels) and can give an 8-bit value for each "spot" (image of an LED), either as a peak reading or an average. I have a variable threshold above which values are determined to be part of a "spot." With the webcam imaging the entire array, I perform a calibration with the LEDs fully lit, which allows the software to know the X/Y location of each LED in the frame. I can then determine the value of any pixel within a given "spot."

Question 1: I'm trying to establish a transfer function between the LED spot illumination (candelas) and the video image data. This is clearly affected by Exposure and Gain settings, as well as distance from camera to LED(s). I can calculate the luminous emittance (lux) for an LED at the camera, but don't know how to calculate the resulting 8-bit value in the video image as a function of gain and exposure.

Question 2: Does anyone make a webcam with variable wide angle (zoom) capability? Or a regular digital camera with webcam streaming capability? I've looked, but haven't been able to find the latter, which seems strange, because most use a USB interface to upload still or video images, and it would seem pretty easy for a manufacurer to add webcam capability to a normal zoom-lens camera.

Would someone at Logitech be able to shed some light ?

Charles

I can't really answer your question on the transfer function but one thing I can recommend is to disable auto-exposure and set exposure and gain to a known value. That should make the values reproducable and allow you to derive the proper thresholds by experiment.

>disable auto-exposure and set exposure and gain to a known value.

Thanks, Martin. I have built a pegboard with 25 LEDs that can be adjusted for brightness, so looks like I will have to go this route.

Regarding gain and exposure settings, is there a better way to set these than just using the Advanced Settings panel sliders?

Charles

Yes, there is. You can programmatically set the exposure time using DirectShow. There's actually an FAQ on this because it's a little trickier than it could be:

Questions about accessing the exposure time through DirectShow

Let us know if you have any additional questions.