The correct height is determined by the uniformity
Maybe, if you have a really shitty fixture. Using half a dozen or more evenly spaced Samsung 4' strips in a 4'x4' area is going to get very good uniformity if not powered in parallel. They can be lowered to inches above the canopy. How many inches requires a PAR meter or spectrometer.
And if you want to know PPF you can estimate it by using a lumen to par conversion factor.
Actually with that calculation he just posted you can. LOL
You can but an estimate is just that, only a worthless estimate. So no.
That's basically what those cheaper PAR meters do anyway.
No. Lux is estimated from ambient light sensors (e.g. photo diode) , not the other way around. They either use an optical filter before the sensor or have an on-chip Lux conversion table after the sensor.
A PAR meter cannot use the same circuitry as a Lux meter. A 550nm photodiode is all that is need to make a cheap Lux meter. Here is a schematic for a Lux circuit using less than $5 in parts. The key part being a 550nm Photodiode.
Cheap PAR meters (ie < $1000) are woefully inaccurate and plants don't care if you are a bit off anyway.
Not true! You can get a very accurate spectrometer well under $500.
Here is a $220 Mini-Spectrometer Head with very accurate readings from 340nm to 650nm. 256 pixel CMOS linear image sensor with 15nm resolution. It needs to be mounted on a PCB with a connector for its serial port. Can be connected to a $10-15 Arduino.
There are also Ambient Light Sensors (ALS) using photodiode arrays for simple and fairly accurate measurements.
I am betting I can make a highly accurate PAR measurement tool for under $100.
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Typical 550nm Photodiode SPD curve
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Ambient Light Sensors
The above photodiode circuit can be reduced to a single chip, called an Ambient Light Sensor (ALS) costing less than $1. The above photodiode response is shaped to the human eye response curve either an optical filter, logic tables, or computational conversions.
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Ambient Light Sensor with Lux Optical Filter
The optical filter shapes the photodiode response to match human eye response.
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There is NO way to use a Lux circuit to measure PAR.
A PAR circuit requires multiple photodiodes with a variety of wavelengths.
An inexpensive PAR circuit will use an ALS with an RGB or RGBW photodioode array.
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Red, Blue, Green, and White sensors can make a very decent PAR meter.
Or an excellent Lux meter with better than 10% accuracy.
ALS = Ambient Light Sensor
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After that you are into spectrometer territory.
A spectrometer diffuses (with prism or diffusion grating) the light and measures from UV to IR and beyond.
Where the detector is typically a Photodiode or CMOS Linear Array sensor like what is used in a scanner or copier with 128-2048 sensors.
The higher end spectrometers will use a CCD sensor.
There is a very inexpensive way to use a camera (e.g. web cam or mobile phone) to capture the diffused (prism) output. Then analyze the image and convert it to an SPD chart.
This is a Kickstart funded ($110,000 pledged) Public Lab project that sells a cardboard spectrometer that uses a piece of a DVD as the grated diffuser and a web cam as the CCD sensor. Sells for $47.
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I am currently working on a design using a six color photodiode array using violet, blue, green, yellow, orange, and red. It will be a very inexpensive, but fairly accurate, for the measurement of PAR positioned somewhere between a PAR meter and spectrometer.
The cost would be less than a PAR meter. About $50-$100. Basically you use a PC or laptop to read the six color values from the sensors and use software to calculate the PAR. Each color is very easily calibrated.