How to Use a Cheap Lux Meter to Increase Yields

by Nebula Haze

Note: This tutorial doesn't work for LEDs. A lux meter is only effective at measuring light levels for MH, HPS, CFLs, T5s, and fluorescent lighting. Unfortunately, a lux meter is not a good tool for measuring light levels of LED grow lights when it comes to growing cannabis.

Did you know you can get more out of your grow lights by measuring your light levels with a lux meter?

Get a lux meter on Amazon.com!

Have you ever wondered if there was some way to measure how much light your plants are receiving in your indoor cannabis grow?

Well there’s a solution – a lux meter! Lux meters measure brightness. “Lux” is the measure of how many lumens per square meter being received at a point in space. Lumens is a measure of light. So in other words, a lux meter can tell you the amount of light received at any specific point in your grow area.

How does a lux meter help you?

  • Bigger Yields! – Plants which get more light grow bigger and faster, producing bigger yields. A lux meter helps you find the “sweet spot” for your grow light so your plants can get the most possible light in your current setup.

  • Test Your Bulbs – You can use a lux meter to find out when you need to get new bulbs. MH/HPS bulbs produce weaker and weaker light over time, so they need to be replaced every few grows. A lux meter can help you know exactly when to replace your stuff.

  • Healthier Growth – You can find out when certain parts of the plant are getting too much or not enough light. Lux meters can also help you avoid light burn caused by too-high levels of lux.

  • Better Than Your Eyes – It can be difficult to just look at a space and know exactly which parts are getting the most light. We try our best, but a lux meter can often show you small adjustments that make a big difference.

My current grow is the first time I've used a lux meter. For this grow I was using a 250W HPS light for the first time. I wasn’t familiar with how far to keep the light away from my plants. Luckily I had a lux meter to give me definitive answers. I started testing light levels and how they vary with the possition of the light distance and the plants.

I found out that 11-12" was the best distance to keep my 250W light from these plants

2 week old auto-flowering cannabis plants under a 250W light

And I kept testing lux levels throughout their life!

As the plants grew, I kept checking to make sure they were all getting the same amount of light. After my plants hit the flowering stage and started making buds, I found out that I could get more light to ALL parts of all the plants by moving everything 3 inches to the right. How crazy is that? There's no way I could have figured that out on my own just by looking at the plants. It all had to do with the shape of the plants and how the colas were making shadows compared to their position underneath the light.

Here's a quick video showing you what I mean:

So a lux meter gave me the tools to get bigger yields by getting more light to my plants. The best part is I didn’t have to change my grow light – I was able to get better performance out of the same lights with different positioning.

How much lux do you want to deliver to your plants?

Quick Guide – Lux Levels for Optimal Cannabis Growth

Life StageMaximumGoodMinimum
Vegetative70,000 lux40,000 lux15,000 lux
Flowering85,000 lux60,000 lux35,000 lux

< 15,000 lux – sparse or "stretchy" growth – plant isn't getting enough light
15,000 – 50,000 lux – good amount of light for healthy vegetative growth
45,000 – 65,000 lux – optimal amount of light for cannabis plants in the flowering (budding) stage
70,000 – 85,000 lux – a lot of light, some strains do okay at this light level, but some plants lose their top leaves early under this light intensity, especially plants that are not resistant to heat/light (like many indicas)
> 85,000 lux – at this light intensity, you've hit the plant's "saturation point" which means your plant can't use all the light (be careful of light bleaching!)

What Happens If I Give My Plants Too Much Light?

The amount of light you get from direct sunlight on a bright, sunny day is 40,000 up to about 100,000 lux. At the high end there's more light than the plant can use. Imagine a cannabis plant in a pot in the desert – the direct sunlight would be too much. 

When growing cannabis, you don't really get any additional gains by adding more light to get over 85,000 lux. Not only is it the extra light wasted by your plants, too much light can actually give your plants unsightly light burn or cause it to lose its leaves early!

After my experience with this handy little tool, I highly recommend getting a lux meter, especially if you've got smaller lights like CFLs or a 250W HPS, or if you're not as familiar with the grow lights you're using. A lux meter lets you compete with cannabis growers who have bigger lights! 

Here's the Lux Meter I use…

Use a Lux Meter to measure light levels
Get this Lux Meter on Amazon.com!

A Lux Meter is super cheap! Get more out of your grow lights!

 
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