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Optics
The photometer is the part of Wavie that turns a colour into a number.
A clear cell, the cuvette, sits between two small boards. On one side is the LED array with three LEDs; on the other, facing it, is the light sensor. The cuvette sits in a black printed holder, with a cap on top and the stirrer below.
| Part | Detail |
|---|---|
| LEDs | 540 nm (green), 615 nm (orange) and 880 nm (near infrared) |
| LED driver | AW9523B constant-current driver, with 8-bit brightness control for each LED |
| Light sensor | TSL2591, with two channels: visible plus infrared, and infrared only |
| Cell | Up to 3.5 mL |
| Mixing | A small fan motor under the cell drives the stirrer |
Only one LED is ever on at a time, so each reading belongs to one wavelength.
- Dark reading. With every LED off, the sensor records the light that reaches it anyway. This is subtracted from everything that follows.
- Reference, I₀. With plain sample in the cell, the chosen LED is switched on and the sensor records how much light gets through.
- Reading, I. After the reagent has reacted, the same LED is switched on again and the sensor records the light again.
Absorbance is then
A = −log₁₀( (I − I_dark) / (I₀ − I_dark) )
A darker colour lets less light through, so I falls and A rises. Each test turns A into a concentration with its own calibration.
Because the reference and the reading are taken through the same cell, with the same LED, minutes apart, most sources of drift cancel out. To keep it that way, a reading and its reference get the same LED warm-up and on-time.
The TSL2591 has two channels. The 540 nm and 615 nm readings use the visible-plus-infrared channel; the 880 nm reading uses the infrared-only channel.
Its sensitivity (gain) and exposure time are adjustable: gain from 1× to 9876×, exposure from 100 to 600 ms. For a test, both are fixed before the reference is taken and kept the same for the reading. Changing gain between the two would add the sensor's own gain error straight into the result.
The LED driver controls current rather than voltage, which keeps each LED's brightness steadier as it warms. Each LED's brightness is set in firmware, not by choosing a resistor, so it can be trimmed for each unit at calibration and stored with that unit's reference readings.
The driver is wired so the LEDs stay off when the power comes on, until the firmware sets them.
Different tests produce different colours, and each 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.
Each unit's dark readings and reference levels for every wavelength are measured at calibration and stored on the unit, together with the LED brightness settings. The dark reading is also measured again at the start of each test.