CactiGuide.com

Just a quick note about growing under lights.
Light frequency and light output are always quoted from a spec sheet. All that is great on paper but in reality it is not correct. When a manufacture says a bulb output is 5000 lumens, that is measured within an inch of the bulb. Plants are never placed 1" away from a bulb. Usually 12" to 24" is the norm. At this distance the actual Lumens drops considerably. 10,000 Lumens or LUX is what is needed to match bright light.
I have 2-4 bulb, 4 foot long fixtures. both are outfitted with T-5 grow bulbs. I use both red and full spectrum. At 24" away I have 12,000 lux.
That will replicate shaded sun.
I see many pictures posted with light set-ups and most appear very low in light output. Very bad for your plants if you are relying on the lights to help with plant growth.
https://play.google.com/store/apps/deta ... lightMeter
This is the light meter I found that works well on my Samsung Galaxy Phone. The front camera is accurate. I compared the readings to a older light meter I used for film photography and they were very close.

The inverse square law of lighting is well known to photographers setting up portrait lights etc. Therefore the illumination intensity of your lights will vary according to the distance from your plants and not just what the rating of the light itself is as Mike explains.

https://www.bing.com/videos/search?q=in ... ORM=VDRVRV

However with light sources that produce heat do not get them too close so they burn the plants.

The inverse square law is only accurate for a point source with no reflection. With multiple or broad light sources and multiple reflectors the geometry becomes far more complicated.

Yes the inverse square law is just a rough guide for placement of the lights. A Lux Meter is a better guide and many photographic exposure meters can read Lux, but with meters in cameras now few have them. However with coloured non full spectrum LED's I don't know how accurate meters intended for natural light will be?

The smartphone Lux Meter Mike found may be the answer.

This article reports the poor results of phone apps compared to a proper überprofessional meter for measuring light from white LED lamps.
"Therefore apps are unfortunately not really of any great assistance for measuring illuminance and not even any use to obtain a general idea of the illuminance value. On the contrary: They lead the user in the completely wrong direction."
https://www.dialux.com/en-GB/news-detai ... lluminance

The light meters that are designed for measuring ordinary LED lamps are dear enough but the ones for measuring single-coloured LEDs are about twice as expensive. Some of those have switching for each of 9 colours, including purple.

My phone and App combination is very accurate. I'll stick to it.
Thank you.

mikethecactusguy wrote: ↑Sat Apr 10, 2021 5:05 pm When a manufacture says a bulb output is 5000 lumens, that is measured within an inch of the bulb.

10,000 Lumens or LUX is what is needed to match bright light.

I believe you misunderstand what a lumen is. It's a measurement of the total visible light emitted by a light source.

Your light measurement device however is not measuring total light output, but rather the amount of light that hits a sensor. This is measured in lumens over an area. In SI units this is lux, or lumens per square metre.

Lumens and lux are not the same thing.

Your right . Lumens and LUX are not the same. Yet. One Lumen is equal to one LUX. For what 95% of the members on this site need to know, it's close enough to help teach them what is required to properly illuminate a growing space. All I was trying to do was help those people, not right a technical paper.

mikethecactusguy wrote: ↑Wed Apr 14, 2021 11:36 pm Your right . Lumens and LUX are not the same. Yet. One Lumen is equal to one LUX. For what 95% of the members on this site need to know, it's close enough to help teach them what is required to properly illuminate a growing space.

Sorry Mike, but this doesn't make sense.

Lumens are a measure of the total amount of visible light, lux is a measure of the light falling per unit area, i.e. lumens per square metre.

If I have a 10,000 lumen light source and use all of that light to evenly illuminate 1 sq m, the light intensity will be 10,000 lux. (In a theoretical world with 100% perfect reflectors etc).
However, if I use all of the light from the 10,000 lumen light source to evenly illuminate 10sq metres, the light intensity will only be 1,000 lux.

Lux: The amount of light that is cast on a surface is called illuminance, which is measured in lux. This can be thought of as light intensity within a specific area. Lumens: The total output of visible light from a light source is measured in lumens. ... One lux is equal to one lumen per square meter (lux = lumens/m2).Aug 30, 2017

I use my phone to measure light intensity, but I do not expect it to show me an accurate value. I just use it to compare, I now know what light intensity value (on my phone) different plants need, so when I for example get a new light I use my phone to put it up in the right distance to where the plant grows.
Now of course one problem with this approach could be that different lamps emit slightly different spectra of light for which my phone might have different sensitivities, but it has worked good enough so far (in combination with observing how the plants react to the light intensity).