Kintronics News: Understanding Resolution and Brightness

Understanding Resolution and Brightness
The more you know the easier it is to select the right camera

To select the right camera for your application, it is important to understand the specifications that are used. The resolution of the camera will determine if you can recognize a persons face, or if you just can tell if there is someone there. The light sensitivity of the camera is also important. Most cameras will work well in normal room lighting; careful review of the camera specifications will determine how well it works 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. Of course, the monitor has to be much higher resolution than the camera being measured for this to work. 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 sensor to detect the video. The number of pixels available in a CCD Camera determines resolution; the more pixels, the higher the resolution. For example, a camera image sensing chip (monochrome) with 400 pixels across could register one white line on one pixel and the black space between the lines on the second pixel, show the next line on the next pixel, and the next space on the following pixel and so on. Thus, a CCD chip with 400 pixels across could see 200 pickets and 200 spaces for 400 lines of resolution.

Well, almost. There is another factor that affects the actual resolution that we can see. Physicists and mathematicians have come up with the Kell factor of 0.7 which says that on the average 70% of the black-white lines will be visible all of the time. We have to multiply our theoretical number of 400 by .7 to get a more realistic number that works most of the time. Thus, a row of 400 pixels gives us only 400 x .7 = 280 lines of horizontal resolution.

Well, almost. TV people measure horizontal lines of resolution in a funny way. They measure it per picture height. The above calculation would be correct if TV pictures were square. Since TV pictures are 1.33 times wider than they are tall, we cannot count all those 280 lines. We are allowed to count only 3/4 of them, so multiplying 280 x .75 = 210 lines of horizontal resolution. That's what is available from 400 horizontal pixels.

Suppose a camera manufacturer advertises an image sensor with 410,000 pixels. How sharp will the picture be? Let's make the calculation. Of the 410,000 pixels, only about 380,000 (92%) fall within the borders of the video image; the rest are off the edge of your TV screen and don't show. The matrix of 380,000 active pixels gets divided into 483 active scan lines (in the NTSC television system). Put another way, the pixels form a box that has 483 vertical pixels along the left edge. This leaves how many pixels per horizontal line? 380,000 divided by 483 = 787 pixels per line. Multiply that by .7 (the Kell factor) and we get 551 lines. Multiply that by .75 (because TV people don't count all the lines, just count the lines that would fit in a square box) and we get 413. Thus a 410,000 pixel image sensor yields 413 lines of 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.

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. We have included some definitions that will help you understand the terms used to define the light sensitivity of a camera. They will help you select the right camera for your application. In defining how bright something is, we have two things to consider.
1. How bright it is at the source - how bright is that light?
2. How much light is falling on something a certain distance away from the light?

Lets' do some definitions now……
We're in America, so we are going to talk about units of measurement that concern distance in feet and inches. So, we will use some terms that folks in Europe don't use. We're going to talk about "foot-candles".

This one's simple. Get a birthday cake candle. Get a ruler. Stick the candle on one end of the ruler. Light the candle. One foot-candle of light is the amount of light that birthday cake candle generates one foot away.

That's a neat unit of measurement. Why? Say you have a lamp. You are told it produces 100 foot candles of light. That means at one foot from the lamp, you will receive 100 foot candles of light.
But here's where it gets tricky. The further away you move the light from what you want to illuminate, the less bright the light seems! If you measure it at the light, it's just as bright. But when you measure at the object you want illuminated, there is less light! A Physics teacher is going to tell you that light measured on an object is INVERSELY PROPORTIONAL to the distance the object is from the light source. That's a very scientific and math rich way of saying, the closer you are to the light bulb, the brighter that bulb is. Or, think of it this way. You can't change how much light comes out of your light bulb. So, to make more light on an object, you have to either move the light closer, or add more lights.

Now, let’s get to LUMENS.

A LUMEN is a unit of measurement of light. It measures light much the same way. Remember, a foot-candle is how bright the light is one foot away from the source. A lumen is a way of measuring how much light gets to what you want to light! A LUMEN is equal to one foot-candle falling on one square foot of area.
So, if we take your candle and ruler, let’s place a book at the opposite end from the candle. If that book happens to be one foot by one foot, it's one square foot. Ok, got the math done there. Now, all the light falling on that book, one foot away from your candle equals both…….1 foot candle AND one LUMEN!

Ah, we've confused you. Let's split off from this and talk about the difference between RADIANCE and ILLUMINANCE.

RADIANCE is another way of saying how much energy is released from that light source. Again, you measure it at the source. Unless you're talking about measuring the radiance of something intensely hot, like the Sun. Then you might want to measure it at night, when it's off.

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 the intensity or degree to which something is illuminated and is therefore not the amount of light produced by the lightsource. This is measured in foot-candles again! And when people talk about LUX, it’s 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. A lux equals one lumen incident per square meter of illuminated surface area. They're measuring the same thing, just using different measurement units.

Pretend you're an old photographer, like O. Winston Link, or Ansel Adams. These two gods of black and white photography (and a print made by either can fetch quite a hefty sum of money these days) used a device called a light meter to help them judge their exposure. (There is another way of judging exposure-that's when someone whispers in our ear at a cocktail party, "You silly twit, your fly's come undone!").

These light meters were nifty devices. You could use it to show how much light was falling on an object, light from the sun, and reflected light energy from every thing else. Or you could use it to show how much light energy was reflected off the object itself.
All this brings back two points.

The first point is if we measure the output of a light at the source that gives us one thing.

The second point is that we use an entirely different unit of measure if we are measuring the results of that light's output.

Summing it all up:
Resolution available determines how clearly you can see the image. Camera resolution varies from under 300 lines to over 500 lines of resolution. As you may expect, the higher the resolution the more the camera costs.

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 well lit parking lot. A rating of .05 has the ability to clearly view an image in a dimly lit area at night. A 0.05 LUX sensitivity can enable a camera to actually see better than the human eye at night!

A good color low light level camera such as the Pelco CC3751H-2, is capable of viewing 0.013 lux at f1.2, 50 IRE. The Pelco MC3651H-2 monochrome camera can view images at a minimum illumination of 0.002 lux at f1.2.

It is also important to know if a camera can operate in a very high brightness environment. For example the Axis2100 ($289) camera can view images as low as 3 lux, but has a maximum light sensitivity of only 10,000 Lux. Since sunlight can be much brighter than 10,000 lux, the Axis2100 is not recommended for outdoor use. Instead the Axis2120 ($995) which has a much greater light range, is recommended. It can operate from 1 to 200,000 Lux.

Need more information? Please give us a call at 1-800-431-1658 or 914-347-2530. You can also email us.

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