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Understanding Camera Specifications

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To select the right camera for your application, it is important to understand the specifications that are used. The specifications include: resolution, light sensitivity, frame rate, color or black/white, lenses, interface тА?network attached or analog video, etc. The specifications will help you select the right camera for your application. For example, the resolution of the camera will determine if you can recognize a person's face, or if you just can tell if there is someone there. The light sensitivity of the camera determines if the camera can be used at night or even in bright sunlight. This article helps you understand the terms used, so you can select the best camera for your application.

Resolution:

TV lines of resolution are classically defined as the maximum number of black and white lines that can be seen on a monitor. Historically, resolution was measured using a test pattern. It provided a series of converging lines with resolution numbers next to the lines. The idea is to look at where the lines converge. When you get to the point where you are just able to see the black-white pattern converge into all black, you have reached the point of maximum resolution. We still use this old TV standard to define how clear a picture appears on a monitor. Probably the most famous American B&W test pattern is the so-called "Indian Head" monoscope pattern. This was used for the first time in 1939 the year that Radio Corporation of America chairman David Sarnoff "introduced" electronic television at the New York World's Fair.

Today's cameras use a CCD or CMOS sensors to detect the video. CCD means Charged Coupled Device, which contains many light sensitive elements or pixels. Each element converts light to electrical signals that are eventually sent to the monitor so that you can view the image. The number of pixels available in a CCD Camera determines resolution; the more pixels, the higher the resolution.

If you ever need to make a quick mental calculation, simply take the number of pixels, divide by 1000, and call it lines of horizontal resolution. Thus a chip with 300,000 pixels would give a horizontal resolution of about 300 lines.

Light Sensitivity

Illuminance, lux, Lumen or how bright is it anyway:
Most cameras will work well in normal room lighting, but we have to take a careful look at the specifications to determine if the camera will work well at night or in bright sunlight.
Foot-Candles
A foot-candle measures how much light is generated from a light source. For example, if you take a birthday cake candle and stand one foot away from it you will see one foot-candle of light. Also note that if you stand further away you see less light and if you stand closer you see more light. The power of the light is inversely proportional to the distance from the light source.

Lumens
A LUMEN is also a unit of measurement of light. While a foot-candle is how bright the light is, a lumen is a way of measuring how much light gets to what you want to light! One LUMEN is equal to one foot-candle falling on one square foot of area.
For example if we take the candle and place a book one foot away from the candle, and if that book happens to be one foot by one foot (one square foot), then all the light falling on that book equals one LUMEN!

LUX
RADIANCE is another way of saying how much energy is released from that light source. Again, you measure it at the source.

ILLUMINANCE is what results from the use of light. You turn your flashlight on in a dark room, and you light something up. That's ILLUMINANCE. Turning on a light in a dark room to make the burglar visible gives you ILLUMINANCE. It also gives you another problem when you note the burglar is pointing your duck gun at your bellybutton.

Illuminance is measured in LUX. It is illuminance measured in metric units rather than English units of measure. To reinforce that, LUX is the measurement of actual light available at a given distance. Here is the definition of a lux (lx): The International Standard (SI) unit of measure for luminous flux density at a surface. One Lux equals one lumen per square meter.
Here are some practical examples. Typical illuminance values are:

1 lx
Full moon

10 lx
Street lighting

100-1,000 lx
Workspace lighting

10,000 lx
Surgery lighting

100,000 lx
Sunshine

What affects the light sensitivity of a camera?

Lens f-number
First you need an efficient lens. A lens with a low f-number (such as f1.4) focuses much of the light from your scene onto the camera's sensitive CCD chip. A lens with twice as high an f-number (f2.8) will pass 1/4 as much light through it, making the camera as sensitive.

Size of the CCD sensor
The camera's CCD chip can be manufactured in a way makes it more sensitive to light. A CCD chip has small squares or elements that are sensitive to light; the more light that falls on these CCD elements, the stronger the signal. The bigger the elements the more light can be collected in a period of time. If we double the size of the elements, we will collect more light. If we increase the size of the elements then the chip size has to be increased. So a ½-inch CCD chip collects more light than a ¼-inch CCD chip. We can also place small micro-lenses on the surface of the chip to concentrate the light and increase the sensitivity of the CCD elements.

SIGNAL-TO-NOISE RATIO (S/N)
The signals from the CCD chip can then be amplified. The more they are amplified, the brighter the picture, but also the noisier the picture. It is like turning up the volume to hear a distant radio station; you hear more music, along with more static and more interference. This electronic noise appears as graininess and color splotchiness in your picture. Improved video circuits amplify the picture signal while adding very little noise. The result is measured using the SIGNAL-TO-NOISE RATIO (S/N) which is measured in dB units. The higher we can make the S/N ratio, the better.

Frame Rate and Shutter Speed
The signal collected on the CCD elements is discharged and transferred to the TV monitor and displayed. The rate that the CCD element is collected is related to the frame rate. Reducing the camera shutter speed also decreases the frame rate. This is another way of increasing camera sensitivity. This allows the light to fall on the CCD elements for a longer period of time. Normally TV cameras make 60 pictures per second. Thus they have 1/60 of a second to collect the light and convert it into a video signal. If the camera made 15 pictures per second, the CCD chip would have 4 times as long to "look" at the picture and absorb the light. It's a little like taking a time exposure with a film camera. Switching to 15 frames per second may quadruple the camera's sensitivity, but it will smear the picture more when objects move (just as it does with a film camera).

Lens Type
A wide angle lens (or a lens that is "zoomed out") captures light from all over the scene. A close-up lens (or zoomed in lens) catches the light from only a small part of the scene. Naturally, a lens that is zoomed out gives a brighter picture than one that is zoomed in. Normally you do not see this difference but in very low light situations, the focal length of the lens (how far it is zoomed in) affects the camera's sensitivity.

Summing it All Up:

Resolution available determines how clearly you can see the image. Camera resolution varies from under 300 lines to over 1000 lines of resolution and depends on the number of elements in the CCD or CMOS chip. As you may expect, the higher the resolution the more the camera costs.

Light sensitivity is measured in LUX units. The LUX measures how much light is seen by the camera. The less light available, the more sensitive the camera has to be. Black-White cameras can "see" images in darker conditions than color cameras. Color cameras operate in low light conditions by slowing down the lens. This is like keeping the lens open on your still camera for a longer period of time. For example: A camera that sees down to 1 LUX, has the ability to see an image during early evening hours or just before dusk. A camera that's down to 0.1 has the ability to see an image in let's say a parking lot with some lights. A rating of 0.5hasthe ability to clearly view an image in a dimly lit area at night. A 0.05 LUX sensitivitycan enable a camera to actually see better than the human eye at night!

For example, the Panasonic WV-NP472 is a high performance network camera. This network attached camera automatically switches from color, in daytime, to high sensitivity black and white at night. It can operate in color mode at 0.8 lx (0.08 fc), using a F1.4 lens. In black and white mode sensitivity is an amazing 0.1 lx (0.01 fc). Plus, its linear sensitivity can be electronically enhanced up to 32 times. With Super DynamicII capabilities, the WV-NP472 makes it possible to obtain clear images of subjects in extreme lighting conditions.