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What is Lux: Shedding Some Light on Low Light Cameras

What is Lux: Shedding Some Light on Low Light Cameras

Manufacturers love low lux ratings, but consumers may be left in the dark.

You've seen the claims: "Outstanding reproduction in low light." Cameras with low-lux ratings are the norm, and manufacturers seem intent on seeing just how low they can go. Claims of 2 lux, 1 lux, 0.75 lux and even 0 lux are not uncommon. What do these ratings really mean? What is a lux, and just how many do you need?

What is Lux?

Simply stated, the lux is the metric unit for measuring the amount of light that falls on an object, and is the European equivalent of the British foot-candle (or lumen). Specifically, 1 lux equals the amount of light that falls on a one-square-meter surface that is one meter away from a single candle. 10 lux equals the amount of light produced by 10 candles one meter away.

A camera with a 1-lux rating claims to be able to produce an image by the light of one candle that is about three feet away from the subject. Many cameras on the market today can do just that. The problem is that the resulting image may be of very poor quality.

When a manufacturer slaps a 1-lux rating on the side of its camera, it is giving a subjective opinion of what is an acceptable image. While the ratings are more uniform, objective and scientific than they have been in the past, what a given manufacturer considers "acceptable" is probably not up to the standards of the quality-conscious videographer. A grainy, monochrome form is probably not what you had in mind when you purchased your "low light" camera.

Also, since "acceptable image" is a subjective term, lux ratings will vary from one manufacturer to another. Brand X's 2-lux camera may produce a noticeably better image than a competitor's camera also rated at 2 lux. There's no way to know without testing them both. This is changing. The Electronic Industries Association (EIA) has created a new standard (EIA-639) that has wide industry support. This should remove the subjectivity from lux ratings and indeed many companies are already using the new EIA-639 standard. We are crossing our fingers that they will also start writing "EIA-639 Lux rating" on their boxes so we'll know what to look for.

The Nature of Light

Lux is a measurement based on light produced by a single candle. Candles, as well as regular household light bulbs, produce light that radiates uniformly in all directions. As distance from the light source increases, the amount of light that falls on the object exponentially decreases. (Lux = 1/d2, with "d" being the distance from the source.) The fixed amount of light is simply spread over a larger area, so there's less of it to go around. If you double the distance from your subject to the candle, you do not receive half as much light. You receive 1/4th as much light. Quadruple the distance and you receive 1/16th as much. You can see how 10 lux of light at one meter can very quickly become 1 lux.

Your subject will also affect your camera's light gathering ability. Lux is a measurement of incident light, or the light that is falling on the object. On the other hand, your camera sees and records reflected light, which is light that bounces off the object and makes its way to the lens. Lighter colored, reflective surfaces will reproduce better in low-light situations than dark objects will. Just because 100 lux illuminates an object, do not assume that the camera is seeing a 100-lux subject.

Optics

There are a couple of ways to boost low-light performance. The method used will affect the overall quality of the picture. Generally speaking, a camera with superior optics will give you a truer reproduction than one that relies on electronics. The goal of superior optics is to deliver as much light as possible to the imaging sensor (CCD).

The bottom line for grabbing more light is size. Larger lenses capture more light. A typical 37mm camcorder lens is no match for the 10,000mm Keck telescope atop Mauna Kea. Clearly, modern camcorders are also about portability and convenience, so we have to compromise lens size somewhat. And size is only the start.

Optically superior cameras start with a quality lens. Not only will a high-end lens allow more total light to reach the CCD, but the light will also be of purer quality, since lens imperfections and impurities scatter light. Higher quality lenses yield sharper detail, controlled distortion, and colors that are more vibrant. Camcorder manufactures have recognized the demand for such quality and have responded accordingly. Companies not already known for their lens making have begun contracting out with manufacturers that specialize in optics in order to produce higher quality products.

Another factor to consider is the number of CCDs or chips that a camera uses. All other things being equal, three chips are better than one. In addition to added imaging chips, 3CCD cameras have superior optics and superior electronics. They use a prism that breaks light down into its three component colors: red, green and blue. Light passes through the lens, enters the prism, and is then split up and sent to one of three separate processing chips. Blue light goes to the blue chip, green to the green, and red to the red. The prism is very efficient in its duties and little if any light is lost during the splitting of the light.

On the other hand, a one-chip camera uses a striped filter to breakdown the light into the component colors. This filter is not as efficient as a prism. Light is scattered and some is lost, and, in low light shooting, you need to keep as much of the light as possible.

Electronic Enhancement

Single-chip cameras do have the ability to make do with very little light. When there isn't enough light, the camera takes what light it does have and amplifies it electronically. This is a reasonable strategy, but the resulting picture is less than perfect. Amplification of a low-light signal inevitably results in graininess and video noise. The situation is like audio tapes that were recorded at too low a level. When played back on your stereo, you can hear the music, but just barely. When you turn the volume up or amplify the signal, you hear the music better, but you also hear a lot of hiss as well.

The same thing happens with amplified video signals. The camera will amplify what little light it does receive but along with it, you will receive plenty of video noise. The resulting picture will have a grainy, colorless look. Sure, you can see your subject, but it is far from the quality you are after.

Keep these factors in mind when looking at lux ratings. Is a camera rated at 1 lux because of its superior optics or because of electronic circuitry? In the end, a side-by-side comparison is the only true test. In doing so you'll find that optics will win out over electronics when it comes to low-light shooting. Of course, for professional-looking video, it's best to forget low lux ratings and invest in a light kit.

[Sidebar: Pitch Black]

Recently, we've seen a lot of extremely low-light video on the news as videographers have been forced to shoot in no-light situations during the war in Iraq. While the quality of the video is low, it's better than no video at all. Here's how they do it:

  • Infrared - These green-tinted shots are illuminated by IR light only. IR light can be seen and recorded by your camcorder, but it can't bee seen by the human eye, which is important when someone is hiding in the dark trying to shoot you. There are a number of consumer camcorders that can record night-vision scenes and IR video lights to provide invisible illumination.
  • Low Frame Rate - Another option is to reduce the frame rate. Low frame rate shots appear in color, but at only four frames per second (4fps), the video is very stuttery.
Tags:  July 2003
Brian
Pogue
Tue, 07/01/2003 - 12:00am