Dimming HIDs affect spectrum?

I read in a hortilux text that dimming bulbs via the ballast, is not good for the bulbs spectrum output. I did not even suspect this prior to reading. Apparently HID bulbs are not designed to maintain their spectrum when dimmed. Some ballast even have a boost option.

Any thoughts or input on this subject?

I have heard the same thing.. I don't use dimmable ballasts....yeehaw

HID bulbs need a certain amount of heat to achieve their max performance. The temperature also has an impact on lifetime. a to low temperature also reduces the lifetime of a bulb. for example the DE 1000W has the same lifetime when run all its life on 600W as on 1150W! but if you run 1000W DE bulbs at their rated 1000W the lifetime almost doubels!

efficiency wise reducing the power reduces the efficiency, running a 1000W DE at 600W reduces its efficiency of converting electricity to light by +/- 35%

but, by increasing the power from 1000w to 1150w the DE becomes 5% more efficient in converting electricity to light but it has a impact on lifetime.

reducing this heat by dimming always has an impact on the spectrum.

in general HID sodium bulbs (HPS) but also CMH will get less red / orange as you dim them.

for this same reason it is not advisible to cool HID bulbs with a direct airflow. If you cool down the bulb to much more energy will be converted into heat and less into light, the bulb wants to maintain is optimum discharge temp.

using boost functions on CMH or HPS bulbs will make them more red but this can be easily overdone, by pushing these bulbs to overboost a big part of the spectrum will move out of the PAR spectrum end will end up as some far-red radiation but mainly as short waved heat radiation.

A clever reflector hood design such as the ePapillon helps to overcome the overheating of these bulbs when run above their design specs.

Easy7 said:

I read in a hortilux text that dimming bulbs via the ballast, is not good for the bulbs spectrum output. I did not even suspect this prior to reading. Apparently HID bulbs are not designed to maintain their spectrum when dimmed. Some ballast even have a boost option.

Any thoughts or input on this subject?

Click to expand...

It depends upon the HID you're referring to, for example, Philips 315W CMH Greenpower and MasterColor Elite 942 dimmed spectra are below.

I for one really like the idea of ePapillion's Double dPapillion, using one Greenpower (for its greater efficiency of covering watts into PAR rang photons vs. Elite 942) and one MasterColor Elite 942 (for its better spectrum and much greater UV-A and UV-B output vs. Greenpower).

In a perfect world, if cost wasn't a consideration, I would use only Greembeams with MasterColor Elite 942 lamps, but considering a large scale commercial operation the Double dPapillion with one Greenpower and one MasterColor Eilte 942 is a very attractive option (albeit considerably less ideal uniformity of radiation over canopy vs. Greenbeams). Another excellent optoin for large scale grow operations is a mix of ePapillion (DE HPS) and Double dPapillion, simply for the issue of start-up costs (due to price of CMH luminaires).

Note: The spectroradiometer used for these data isn't top of the line, it's merely entry level, so these data cannot be considered very accurate, but they are a good representation of effects on spectra by dimming.

MasterColor Elite 942 (ePapillion calls this the "daylight" lamp, I believe):



Greenpower (also known as MasterColor Eilte Agro 930):

In general, I always look at an electric item being make for a certain voltage and watt as being the most efficient only with that requirement. If a ballast can be dimmed, I only use a bulb made for that dimmed watt setting. A 12 volt motor may run with 9 volts, or even 24 volts, but it isn't good for it and will fail sooner. Yes, we are talking about bulbs here, but it it was made for X watts, giving it less or more will change the light and cause more wear and tear on the bulb.

Intensity trumps spectrum, period. Even if there is a bit of spectral variation when dimming, it isn't enough to worry about. Were it otherwise, companies like Gavita wouldn't offer the option of dimming.

Nothing but good responses here. I'd like to add this chart, it comes from data pulled from 1000bulbs.com for Plusrite bulbs. I don't recommend the bulbs, they are just what I collected the data on.


If you just need 400 watts worth of HPS, you are probably a little better off getting a 400 watt HPS than dimming a 600 watt bulb and ballast. However, if you ever want more than 400 watts, you will wish you had gotten the 600, and that is what I would probably do. Bulbs are cheap so I'd run 600 watt bulbs at 400 watts if that is what I needed and replace them when they degrade or fail. Peace!

Thanks for chiming in on this subject, everybody. Seems like dimming is just a marketing gimmick. Perhaps someday they will have bulbs that can be dimmed, I have my doubts.