Sanjay's Lighting Guide - Frequently Asked Questions

What is this site all about?
Over the last several years I have been testing metal halide lamps and ballast combinations in an effort to get some quantitative data on the light output from these lamps and ballasts. The emphasis is not on determining what is the best lamp, but rather to determine what the lamps are actually doing. Once this is known, then we can begin to answer the following questions:
Given a bulb/ballast/reflector combination - What light levels and distribution can I expect ?
What are the differences between different bulb/ballast/reflector combinations ?
Does a particular combination meet my needs (coral needs)?
How I can design around the limitations/ strengths of a particular combo?
The site currently does not have data on the reflectors, but this information will be added in the near future.

What data is available on this site?
This website has the following data available for over 900 metal halide lamps and ballast combinations:
Plot

The spectral plot shows the amount of power output by a given lamp at different wavelengths. The X-axis shows the wavelength and the Y axis shows the power density. For example, from the spectral plot above, we can see that the Ushio 400W lamp running on a Magnetek ballast has 0.40 Watts/m^2 of power density at 420 nm. Performance data for the lamps and ballast, which includes the power consumption, PPFD, CCT, and efficiency.

SPECTRUM	Lamp Name	Ballast Name	Shield	PPFD	CCT	Power	Volts	Amps	Efficiency
plot	Happy Reefing 400W 10000K DE 1	PFO 400W HQI	N	276	8000	548	123	5.42	0.5053

Lamp name – Identifier used for the lamps (brand name – Wattage – Color Temp as sold – Type of lamp, single ended (SE) or double ended (DE) – Designator for the number of lamp tested, so 1 means sample #1, a 2 would mean sample #2, etc. Ballast name – Ballast used to test the lamp
Shield – (N or Y), indicating if the lamp was shielded with a glass shield when the data was collected. Double ended lamps are usually required to be used with a shield, so data was collected under both shielded and non shielded conditions. PPFD – Photosynthetic Photon Flux Density as measured. see 4 below CCT – Correltated Color temperature, as measured. see 5 below Power – Indicates the actual power consumed in Watts when the lamp was running as measured by a true RMS Watt meter. This is what your electric company will charge you for if you were to use this ballast and lamp combination. Volts – measured the actual volts going into the ballast when the test was conducted Amps – measured the actual amps going into the ballast when the test was conducted. This number can be used to estimate how much amps your wiring will be subjected to under steady state conditions. Remember that this number can be higher when the lamp is starting up. Efficiency – this is ratio of light output to input power (PPFD/Power). Provides an indication at how efficiently power is converted to light.

How does the wavelength correspond to color?
The visible light spans the wavelengths from 400 nm to 700 nm. Visually this approximately corresponds to the following colors:

Violet - 400 to 440nm

Blue - 440 to 490nm

Green - 490 to 540nm

Yellow - 540 to 590nm

Orange - 600 to 650nm

Red - 650 to 700nm

What is PPFD?
PPFD stands for Photosynthetic Photon Flux Density, and is the technically correct term when PAR (Photosynthetically Available Radiation) is measured as the number of photons of light falling on a given area in a given amount of time.. The units are micromoles per meter square per second. 1 micro mole is = 6.023X1017. So for example, when you see a value for PPFD=200 it means that there are 200X6.023X1017 photons falling on a 1 meter square area in one second.
What is difference between PAR and PPFD?
PAR is the Photsynthetically Available Radiation which is all radiation between 400-700 nm wavelength range. It can be measured in several units. Just like distance can be measured in feet, meters, etc. PAR can be measured in different units used to measure radiation. When PAR is measured by the number of photons falling on a given area in a given amount of time, this is called PPFD and the units used are micromoles per meter square per second.
What is CCT?
Correlated Color Temperature (CCT) which is defined as the value of the temperature of the black body radiator when the radiator color matches that of the light source. So when you see a lamp labeled as 10,000K what it implies is that the color produced by the lamp is similar to color produced by a black body heated to 10,000 Kelvin.
Why are these numbers different from what I have seen from other people’s data?
The PPFD measured is a function of the distance at which the data is collected. To compare 2 different readings you have to make sure that the distance at which the measurements are made is the same. Since PPFD measures light falling on a given area, as we increase the distance from the source to the measurement plane, the same amount of photons get spread over a larger area.
At what distance are these measurements made?
All measurements are made at 18” from the centerline of the lamp to the top of the sensor dome.

What instrument is used to make these measurements?
Licor Spectroradiometer LI-800

Where can I go to get more information?
The following articles from my Facts of Light series provide more detailed information on lighting for aquarium hobbyists.

Part I: What is Light?

Part II: Photons

Part III: Making Sense of Light Measures

Part IV: Color Temperature

Part V: Everything You Need to Know About Metal Halide Lamps and Ballasts