TSL1401 - best LED for lighting the scene

rbtying

Dear all,
I would like to know what color/type of LED would be best for lighting up the scene for the TSL1401 line sensor on the linescan imaging sensor daughterboard. The datasheet shows maximum response around 800nm, but some browsing around the forums leads me to believe that there is an IR-cutoff filter in the lens (and so the 850nm IR LED light would be cut off).

Any response would be greatly appreciated - thanks in advance.

Phil Pilgrim (PhiPi)

The lens supplied with the TSL1401-DB does have an IR-cut filter, but I don't know what the cutoff wavelength is. Unless the color of your subject dictates otherwise, you'll get the most bang per watt/dollar with "super red" LEDs. For close-up front-lighting work, I would recommend a pair of P4 ("Piranha" package: 7.62mm x 7.62mm) LEDs, one on either side of the lens. A 70-degree beam pattern should be about right. Here's an example:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=160-1505-ND

You can also get these kinds of LEDs with oval lenses, which spread the beam in one axis more than the other. This is ideal for linescan illumination. But sourcing such LEDs can be a hit-or-miss proposition.

-Phil

rbtying

Would these be good? (http://www.sparkfun.com/products/528)

They're also listed as Super Bright Red, and I'd just put a bunch of them together to make a light bar.

I'd also like to know whether there is a significant response from this sensor at ranges of ~36-48" (white vs. black line), if brightly illuminated.

Phil Pilgrim (PhiPi)

The reason the Sparkfun LEDs have such a high millicandela value is that the beam is concentrated in a 10-degree cone. You will get more even illumination by overlapping LEDs that have a wider beam angle. The reason I like the P4 LEDs for close-up work is that there are virtually no hot spots in their beam pattern.

As to sensor response at 36-48" (and well beyond), there should be no problem at all if the subject is illuminated brightly enough and has a high contrast with its background (such as your white vs. black lines). Just remember, though, that a thin line up close will appear much thinner at a distance and may well disappear entirely if it doesn't span more than one pixel in the field of view. As a general rule of thumb with the supplied lens, the size of one pixel in the field of view will be about 1/128th the distance to the subject. So at 32" away, for example, each pixel will cover about a 1/4" square.

-Phil

rbtying

Thanks for the help! I will look into the P4 LEDs for illumination. The reason I suggested the Sparkfun ones was because I already have a few of them, but if their beam distribution is an issue then I'd rather go with your recommendation.

The line I'm trying to find is about 2" wide, so it should be visible enough, right?

Phil Pilgrim (PhiPi)

If the lens is properly focused, you should have no trouble seeing a 2"-wide line at a four-foot distance. It will be about five pixels wide. If you're using the TSL1401-DB with the Mobo or Prop Backpack, you can use the monitor program to adjust the focus for maximum sharpness (i.e. best contrast).

-Phil