Easily.
If you are trying to reproduce an experiment by following a protocol in a published journal, they might be specifying something like an optical density at 600 nanometers of 2.
That would mean the sample blocked 99% of the light (optical density is measured in log 10 of the ratio of light in to light out). An optical density of 3 means 99.9% of the light is blocked. Basically, you count the number of nines.
So you put your broth containing no bacteria in the spectrophotometer, and take a reading, noting the value at 600 nanometers (deep red). Then you put your sample of bugs and broth in the light path and take another reading.
Divide the second reading by the first.
Now a spectrophotometer such as the one under discussion has no way of delivering RAW images -- web cams only give you JPEGs. So you will only have 256 levels to work with. If your sample is OD 2, and your blank gave you a reading of 250, your sample will give you a reading of 2. As you can see, using the webcam device will not be able to tell you much about higher densities.
A device like the one on my website could be modified to use RAW images, giving you 14 bits per pixel. So the blank might give you a reading of 16,300, and the sample would read 163 for OD 2, and 16 for OD 3.
You can extend the range if you have a dimmer on the light source, and you have calibrated the dimmer with the spectrophotometer, so you know what settings give you 100% of the light, 10% of the light, and 1% of the light. A red LED and some resistors on a rotary switch would do the trick. Now you take a reading of the blank using 1% of the light, and a reading of the sample using 100% of the light, and you get to OD 4 using the simple webcam version of the device.
Generally you might be looking for the log growth stage of your sample. In that case, anything in the 600 nm to 650 nm range will do nicely, and you don't need to bother with a spectrophotometer. Use a cheap red laser and the resistors, and a cadmium sulfide photocell and a cheap multimeter. The red laser has a nice sharp spectrum peak and little else, and it is probably much sharper than the spectrophotometer is (sub-nanometer).
You could build a probe out of some plastic optical fiber. The laser goes into one bit of fiber, and exits into the sample. A millimeter from the exit is another fiber end, conducting the light up to the photosensor. You dunk the probe into the beaker of bugs and broth, and let a microcontroller monitor the optical density and look for logarithmic change, whereupon it beeps or lights an LED. You can run the laser (or a red LED) from different output pins on the microprocessor, each one giving you 10 times more light than the last one, through the use of different resistors attached to each one. A microcontroller like the Ti430 Launchpad (cost is $4.30 for the USB enabled development board) has a 10 bit analog to digital converter, and 16 output pins, giving you a possible accuracy of 26 bits, assuming you can control the brightness of the LED using the 16 bits and get all of the range. That gets you to OD 7, where 99.99999% of the light is blocked.
But if you only need OD 2 or 3, the 10 bit ADC and maybe three output pins will easily get you there. And it could send the data back to your laptop computer for graphing or recording by printing to the USB port in humanly readable ASCII.
On Mon, Feb 4, 2013 at 4:41 PM, Dakota Hamill <dkotes@gmail.com> wrote:
Could this be used to measure turbidity for optical density measurement?
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