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How Much Yield are Your Lights Losing You?

How Much Yield are Your Lights Losing You?

Alex Grady

Like anything electronic, grow lights become less effective over time. 

Reflectors lose reflectivity, and lamps degrade. The resulting loss in light can have a big impact on yields. 

How much yield are you losing?

Why Do Lamps Degrade?

Light output from HPS and MH lamps gradually declines over time.  

The rate at which light output decreases depends on:  

  • Physical changes in the lamp (e.g. electrode deterioration)
  • Changes in the chemical balance of the metals in the arc tube
  • Changes in the way the ballast performs.  

Why Do Reflectors Lose Reflectivity?

Over time, and through exposure to heat from your grow lamps, reflector surfaces oxidise.  

The Dutch lighting manufacturer GAVITA shows that even high quality micro aluminium reflectors will lose 1% of reflectivity for every 1000 hours of light operation.  

On top of this, reflectors often become covered in dust and dirt, residue from foliar sprays and mineral deposits.  

How Does Light Loss Affect Your Yields?

Overall light loss is DIRECTLY proportional to yield loss.  

A 5% reduction in light output from the lamp and reflector will result in a 5% drop in yield.  

Note: With most reflectors, around 50% of light hits your reflector. This means a 5% drop in reflectivity costs you 0.5% of your yield.  

Replace your lamp and reflector together and your Gavita will emit as much light as when it was new!

How Bright Is Your Bulb?

Most manufacturers tell you how much light output is lost over a year. However, one grower will use their lights way more than another in a year.  

So how many ‘hours’ is a year?  

‘A year’ is based on a standard growing cycle of 1172 hours.

1172 hr Growing Cycle 
4 Weeks Veg 18hrs on  = 504 hrs
8 Weeks Flowering 12hrs on  = 672hrs
    TOTAL = 1172hrs

Of course, each lamp is different.  

Our Gavita Light Systems maintain 96% of light over a yearthat’s incredible.

As a rule, HPS (high pressure sodium) lamps are slower to degrade than MH (metal halide) lamps are.   Take a look at the graph below.

You can see that the Sylvania Grolux’s MH lamps are faster to degrade than their HPS lamps.

How to Test Light Output


You can use a quantum meter or integrating sphere - also known as an Ulbricht Sphere.

This’ll tell you how many photons your grow light is currently emitting (measured in micromoles).  

This is known as thephotosynthetic photon flux (PPF) and is measured in micromoles (µmol/s)


Read more on measuring light output here.  

How Reflective is Your Reflector?

This depends entirely on your reflector and your grow room set up. But on average, you can expect:

  • In a clean grow room:   5% reflectivity loss after 12 months
  • If you use a humidifier or Hotbox Sulfume:  10-25% loss in reflectivity after 12 months  

Remember, with a working reflector, plants will get 50% more light than if you use a bare lamp on its own.  

4 Ways to Prevent Light Loss

It's all about keeping your reflectors and lamps clear of debris and build ups.

1. Use a digital ballast

  • Your ballast regulates how much power is sent to your lamp.  

    Magnetic ballasts aren’t as good at doing this. Every time you switch your grow lamp on, there's a sharp power surge that can speed up lamp degradation over time.  

    With a digital ballast, this doesn’t happen. Instead,  lamps are gradually brought up to the right output.  

    More on digital vs. magnetic ballasts.

2. Use an RO Filter

  • If using a humidifier with unfiltered water, mineral deposits build up on your reflectors and lamps.  

    You'll lose up to 10% of reflectivity after just 3 months.

    If you use a humidifier, always use an RO filter to reduce bicarbonate (hard water) build ups.

    Read more on water quality here.

3. Clean your lamps and reflectors

  • You’ll never get them back to their original state, but you can remove some of the dirt, dust and bicarbonates by cleaning your lamps and reflectors.  

    Use a soft microfiber cloth and mild acidic solution.   

    A lot of growers use Oxy-Plus to clean their grow room equipment (including reflectors) between crops.

    Don't touch lamps or reflectors with your bare hands - always wear gloves.

4. Limit light lost to walls

  • It’s not related to your lamps of ballast, but another way growers lose light is to their walls:    

    • When it is absorbed by your wall 
    • When your wall reflects it to an area where there are no plants  

    To ensure less light is lost:  

    a) Opt for a larger grow room when possible: Because the wall to area ratio is greater - proportionally, there’s less wall for your light to hit.  

    b) Ensure the walls in your grow room are reflective. This means less light is absorbed and more light will be reflected back into your grow space.

When should you replace your lamps?

Very few growers use a grow lamp to its full life for an obvious reason:  

Sometimes it costs you more in yields to not replace your lamps.  

Of course each lamp is different. But on average, you should use each lamp no more than:  

  • 4000 - 5000 hours (HPS)
  • 3000-4000 hours (CDM)
  • 2000 – 3000 hours (MH)
  • 30,000 (Gavita LEP) 

How Often Should you Replace Reflectors?

As often as you need to.

Just remember that at least 50% of light from your bulb hits reflectors. 

So every 1% of reflectivity lost costs you at least 0.5% of your yield.

That's why we recommend replacing them every 12 months.  

Here’s a picture of hundreds of seemingly unblemished reflectors being removed from a greenhouse in Holland: 

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