Using a Cheap Lux Meter as a Plant Light MeterWe use a light meter to measure light intensity. Light intensity plays a very important role in photosynthesis rates. Photosynthesis rate is the major determination in our yields.
To a point, more light = more yield.A light meter only determines how much light there is, not the spectrum.
I showed some spectrum shots below only to illustrate a point of how our eyes and a lux meter perceives light and versus what's closer to what a plant sees.
For everything but LED grow lights, this is close to the readings that we want with a cheap lux light meter as measured at the top of the plant canopy level:
15000-20000 lux____ the lower end of what we want for veg growth
35000-40000 lux____ what we want to try to hit for flowering
75000 or so lux_____ it's pointless to go beyond this level of light intensity, saturation level
We use this information to determine how far away we want the light source from the plant. It's not the case that we should just try to get the plant as close to the light without burning. There are too many types of lights to use that as a rule of thumb as well as too many other variables.
For example, blowing air on the light and plant will allow us to get the plant closer to a light without burning but we in the process could go above the 75000 lux point.
That's just a
waste of energy since we're now saturating the plant.
For
best yields per lighting levels, go with the 35000-40000 lux level. Just accept that or read below on light intensity theory.
This information is not used to determine the
type of light we use.
For example, we generally want redder lights, which have a lower color temperature, for flowering and bluer light, which have a higher color temperature, for vegetative growth. If you want the theory why, read the
photomorphogenesis sections of the
lighting guide.
When using a light meter, it is typically best to use it with the
sensor/meter pointing straight up
rather than directly at the light source. That little white semi-sphere or flat piece of plastic you see with the light meter compensates for this.
What you should take away from this is that a lux meter is accurate enough to give meaningful measurements with white lighting.-------you do not need to read beyond this point, the rest is a lot of theory-------Here's the
conversion charts for using a lux meter as a quantum light meter. Thank you
jcraw69 for pointing this link out to me!
Instead of asking how much light do I need,
spend $20-30 on an Ebay lux meter
and get a good reading. If you use CFLs without reflectors, for example, you'll need twice as many of them.
If in doubt use 75 as a conversion number which is close enough. This means for veg growth with a white light source (more on this below) you want to be around at least 15000-20000 lux (about 200-250uMol), for flowering you want to try to hit 35000-40000 lux (about 500uMol) and the do not go over line for flowering is right around 75000 lux (about 1000uMol which is the photosynthesis saturation point).
Do not use a cheap
analog lux meter. I've tested one type and they were
way off. I haven't tested cheap digital lux meters but I'm pretty sure most are going to be close enough (the analog ones had impedance matching problems with the analog scale so were giving bad readings in brighter light).