Light meters and spectrometers
by /u/SuperAngryGuy of /r/HandsOnComplexity
A quantum light meter is called 'quantum' because their measurement is in the amount a photons hitting a specific point in space per second and a photon is a quanta of light. A lux meter is called 'lux' since they measure luminous flux.
Spectral response curve
Lux meters try to get this spectral response curve (the black curve) and typically use an inexpensive silicon photodiode with a particular filter that rolls off the red end. The photodiode naturally has a blue roll off and this, with economy of scale, allows pretty accurate meters to be made cheaply compared to quantum light meters. That filter is just a cheap greenish piece of plastic with this spectral response.
The high end quantum light meters use a silicon diode with a very expensive spectral response flattening curve and A thin film optical band pass filter to only read 400-700nm light evenly. That's why there is a big price jump in meters prices. These meters also use a digital smoothing filter so the readings aren't bouncing all over the place. If you're serious about lighting you'll get a full spectrum quantum light meter.
For red/blue 'blurple' lighting and for professional or academic use for all lighting, I recommend either the Apogee MQ-500 quantum light meter or the Apogee SQ-520 quantum light sensor. If that is not an option, the Dr.Meter lux meters may be a good alternative, like the 1330B-V. For pro/academic use or advanced hobby use, get the MQ-500 if you are doing more field use, get the SQ-520 if you are doing more lab use and don't need to be portable.
According to Bruce Bugbee, founder of Apogee Instruments and the Director of the Crop Physiology Laboratory at Utah State University, your light meter should never have more than a 5% error over 400-700nm for academic purposes. A lux meter should keep you within 10% error for most white light sources (as per my testing), as long as a rough conversion value is known.
Apogee MQ-500 quantum light meter
Quantum light meters and lux meters are basically worthless for far red lights and far red LEDs. For those you need a spectrometer, a far red sensitive spectral sensor, or something like an Apogee SQ-620 which is PAR and far red sensitive. Red/far red sensors for microcontrollers start at about $25.
Do not use a cheap analog lux meter. I've tested one type and it was way off (the analog ones had impedance matching problems with the analog scale so were giving bad readings in brighter light). These cheap 3 in 1 light meters, pH meters, and moisture meters are worthless.
You generally want the light meter pointing up and down, not at the light source, to get a cosine correct reading. This is a huge mistake I see people make: the white piece of plastic over the sensor gives the proper cosine correction, but not tilting the sensor towards the light will give false readings.
You can calibrate any light meter for PPFD as long as the meter has cosine correction. Most light meters are highly linear; a light meter based on a light dependent resistor would likely not be linear, but silicon diodes are linear to within 1% over 7-10 orders of magnitude.
Your phone is a poor light meter if it has no cosine correction (highly likely does not). I can set up conditions where my Samsung A51 (and Samsung S7) are ten times off a true reading. This is a hardware limitation that can not be corrected with an app. Phones are basically worthless for color LEDs due to the luminous efficiency issue. Based on hands-on experience, I automatically discount all lux measurements done with phones.