Optics

The photometer is the part of Wavie that turns a colour into a number.

Layout

A clear cell, the cuvette, sits between two small boards. On one side is the LED array, a small board with three LEDs. On the other is a light sensor. The cell sits in a black printed holder that keeps stray light out.

Part	Detail
LEDs	540 nm (green), 615 nm (orange) and 880 nm (near infrared)
LED driver	A constant-current driver with 8-bit dimming for each LED
Detector	A TSL2591 light sensor with two channels: visible plus infrared, and infrared only
Optical path	About 10.6 mm through the sample
Cell volume	3.5 mL usable
Mixing	A 25 mm fan drives the stirrer under the cell

How a reading is taken

Only one LED is ever on at a time, so each reading belongs to one wavelength.

1. With all LEDs off, the sensor records a dark reading. This is subtracted from everything that follows.
2. With plain sample in the cell, the chosen LED is switched on and the sensor records the reference intensity, I₀.
3. After the reagent has reacted, the same LED is switched on again and the sensor records the intensity, I.

Absorbance is then

A = −log₁₀( (I − I_dark) / (I₀ − I_dark) )

Because the reference and the reading are taken through the same cell, with the same LED, minutes apart, most sources of drift cancel out.

Why three wavelengths

Different tests produce different colours, and a colour absorbs most strongly at a particular wavelength. Green and orange cover colour changes in the visible range. The near-infrared LED covers methods whose colour absorbs beyond what the eye can see.

Calibration

The brightness of each LED is set in firmware and stored as calibration data for that unit, together with its reference readings. The sensor's gain and exposure time can be fixed for a test, so that a reading and its reference are always taken on the same settings.