How to Measure Light Bulbs

As any frustrated physicist will tell you, light is complicated. And getting it to do what we want is even harder. So designing lighting for residential and commercial purposes CAN be a confounding process. However, with a few tools and a little bit of know-how we can chop it down and make the whole thing a little more approachable. The first step is to know how much light is coming out of a bulb and how that hits the surrounding area – but to do that, we need to cover some basics about how light works.

Some quick vocabulary

First things first – when we’re talking about lighting equipment, there exists a different vocabulary from what most people are used to. So before we continue, we should get a few of the confusing terms straight.

Most notably is the word lamp. In lighting, a lamp is a piece of equipment that, when connected to an electrical power supply, produces visible light. Often called a bulb, these are the most delicate parts of a light.

What most people would call a lamp is actually referred to as a fixture in lighting terminology. The fixture is the body that holds a bulb – it contains the physical pieces that direct and shutter the light.

A filter is a generic term for something that reduces the intensity of a light source. A filter can selectively reduce power by wavelength, producing different colors, or generally, to diffuse the light, making it less offensive to the human eye.

How is light measured?

Light is measured in two ways: in terms of illuminance and irradiance. Both measure the intensity of light, but there is one key difference. Irradiance is a straight measure of the power transferred from the source to the observer – this is measured in watts per square meter. Irradiance is usually used for safety purposes, not for commercial lighting. For measuring bulbs, Illuminance is usually used.

Illuminance is the measure of light energy transferred from a source, but it is altered, according to the luminosity function, to account for the differences in the ways the human eye perceives different wavelengths of light. The common measurements lumen and lux are both measurements of illuminance.

What you are most likely to see on the specifications of a bulb is the unit lumens. As mentioned, lumens measure illuminance. More specifically, packaging for a bulb will usually claim xxxx lumens – this measures the total luminous output of the bulb. Lux, however, measures how well-lit something is, or how much light from the source is making it to the point of observation.

To put things in perspective, to achieve 500 lux (the level in a well-lit office) in a 10×10 room you would need about a 10,000 lumen compact fluorescent lamp(CFL) – which would run at around 20 Watts. For reference, this is about the equivalent of a 65 Watt incandescent bulb. For an area twice the size (roughly 14×14), you would need four times as much light, or 40,000 lumens, assuming it was all coming from a single light source. This is because the power of a light diminishes exponentially as you get farther away from the source.

Measuring the output of an existing bulb

The output from a light fixture can be measured by a piece of equipment called a light meter. If you own a nice camera, chances are it has one built in that you can use for comparison testing, but if you want a numeric output, you will likely have to buy a dedicated device. These can be purchased from photography or agricultural suppliers, usually for less than fifty dollars. But before purchasing, you should always make sure your selected device will cover the range of intensities that you are looking to measure.

If you are planning to replace a bulb in an existing fixture, you will first want to measure the effect of any shades or screens that would reduce the luminosity of the bulb. Such are common in most non-industrial environments as direct light is rather irritating to the human eye. So for a commercial or residential fixture, a good technique is to measure the output of the bulb with and without the shade. Dividing the former over the latter will give you a good approximation of the effect of the fixture.

If you find the ratio of your outputs to be, say 70%, you would then need to find a bulb that is 1.3x as luminous. In the above example, this would mean a 13,000 lumen bulb to achieve the same 500 lux desired in our theoretical 10×10 room.

As we have seen, professional lighting can be complicated, but precision is relative in the world of light. The average living room is lit at about 50 lux, but you are not likely to attract complaints until you get up over a couple hundred. An office can range anywhere from 300-500 lux, and a TV studio is lit at around 1,000 lux. If you can keep the information here straight, you should be fully equipped to tackle any regular lighting challenge you are likely to encounter short of a professional contract.