Everything You Need to Know About Camera Sensors
By Sandy Berger on November 27, 2008


Introduction

The sensor is the part of the digital camera that captures light to create an image. It is analogous to the film in non-digital cameras. Similar to the coating of light-sensitive material on photographic film, the sensor of a digital camera has light-sensitive cells. In this tutorial we will teach you everything you need to know about this important component.

Although there are technical and design differences in digital camera sensors, they all operate on the same basic principles. A sensor has millions of light-sensitive cells or photodiodes on a wafer of silicon. Each of these generates an electrical charge when struck by a particle of light that enters the camera through the lens. A colored filter produces the proper coloration. Then the camera’s processing turns these electrical charges into an image which is then stored on the camera’s memory or storage card. Each photodiode creates one pixel in the final image. This is where the term megapixel comes from. Pixels are the small pieces of information created by light-sensitive photodiodes. Millions of pixels are combined to create an image.

There are two main types of camera sensors: CCD (Charge Coupled Device) and CMOS (Complimentary Metal-Oxide Semiconductor). There are involved technical differences, including the circuitry design, but both perform the same function. The CMOS processors are easier and more cost effective to produce in larger sizes, so they are often used in larger SLR (Single Lens Reflex) cameras. CMOS processors also use less power, so they are commonly used in mobile phones. Most compact point-and-shoot cameras and camcorders use the more common CCD sensor.

Several other sensor types are also available, but they are much less popular. The Foveon X3, is a newer chip that is actually a type of CMOS sensor with a special color processing. It is currently only being used in some Sigma and Polaroid Cameras. Fujifilm also produces some of their cameras with a special sensor called the Super CCD. This is a conventional CCD with individual photocells that are arranged in a diagonal honeycomb pattern rather than the horizontal/vertical grid pattern used in most sensors.

Sensor Size Matters

Although analog cameras had several film formats including those for medium and large formats, the most popular film size was the 35 mm film. So even today’s digital cameras sensors are often compared to 35 mm film. In fact, sensors that are the same size as 35 mm film are called “full frame” sensors. As you can imagine, in terms of processor chips, these sensors are quite large. So the image sensors in most digital cameras tend to be smaller than the 24 mm x 36 mm image area of a full-frame 35 mm cameras. In fact, some tend to be much, much smaller.

To complicate matters for the end-user, there is no unified measuring system for sensors. Some manufactures measure in inches, some in millimeters. Some measure horizontally and vertically while others measure diagonally. For the most part sensors fit into different types. You will hear the terms like 2/3” type, 1/1.8” type and 1 /2.7” type. These are typical sizes that are generally used in CCDs in smaller point-and-shoot cameras. SLR cameras with interchangeable lenses generally use larger sensors. Two of the most common SLR sizes are 4/3” type and 1.8” APS-C type.

The fractions given in naming sensor types are actually larger than the actual dimensions. We are told that this naming convention is a throw-back to the way that TV camera tubes were named in the 50’s. Obviously, these naming conventions should have been discarded long ago, but like a bad egg, their trail continues.

The table below gives a breakdown of these various sizes and their actual vertical, horizontal, and diagonal measurements. Aspect Ratio is the ratio of width to height.

Type

Measurements (W x H)

Diagonal Measurement

Aspect Ratio

2/3” type 

8.80 mm x 6.60 mm

11.00 mm

4:3

1/1.8” type

7.18 mm x 5.32 mm

9.00 mm

4:3

1 /2.7” type

5.37 mm x 4.04 mm

6.72 mm

4:3

4/3” type

18.0 mm x 13.5 mm

22.5 mm

4:3

1.8” (APS-C) type

22.7 mm x 15.1 mm

45.72 mm

3:2

35 mm film

36mm x 24 mm

43.3 mm

3:2


The APS (Active-Pixel Sensor) naming convention seems to be a move away from the “fraction” type of names. However, it is no less confusing because different manufacturers use different APS sizes. The APS-C type mentioned above is a popular size of APS chip, but some Nikon APS-C chips are slightly larger and some Canon APS-C chips are slightly smaller. Canon also has a larger APS-H sensor.

To get a feel for the difference in sensor sizes, see Figure 1 which gives a visual image of the largest compared to the smallest.


click to enlarge
Figure 1: Relative size comparison of full sensor (25mm x 24mm) to 1/1/8" sensor type (7.18 mm x 5.32 mm).

The reason that the size of the sensor is so important can be easily recognized when you envision the millions of megapixels on the sensor. As you may know, or may have read in our Everything You Need to Know About Megapixel tutorial , manufacturers are still trying to entice photographers to buy their cameras by constantly increasing the number of megapixels in the camera. Since the pixels count in the millions (megapixels) you can imagine that to add more photodiodes to a certain sized sensor to create more megapixels, the pixels themselves have to be smaller. The smaller the pixels, the less light they can handle. When the pixels don’t get enough light, the result is what is called “noise.” Simply put, noise is the presence of specks of color that don’t belong in the photograph.

So, all other things being equal, millions of photodiodes crammed onto a small sensor will result in poorer photos than the same number of photodiodes on a larger sensor. If the sensor is larger or the megapixel count is smaller, each photodiode can be larger. So, given the same pixel count, a larger sensor will capture more light per photodiodes than a smaller sensor and produce a better picture.

A small sensor is not entirely bad. It allows the camera to be small and very portable. That is why digital cameras can be so much smaller than film cameras. Smaller sensors are also less expensive, which can help keep the cost of the cameras more affordable. So there is room for both large and small sensors in the photo marketplace, but the consumer should understand that there are tradeoffs with smaller sensors.

Color Reproduction

Each photodiode in a camera sensor detects only the brightness of the light. In order to reproduce colors, each cell in the sensor is covered with a colored filter that transmits only the light in a similar wavelength. For instance, a blue filter will transmit only the blue light. A red filter will transmit only the red light. Filters are created for each of the primary colors: red, green, and blue. They are laid out in a pattern, so that each photodiode will be able to transmit all three colors. Most camera sensors today use a filter called a Bayer mask filter. (Named for Dr. Bryce Bayer of Kodak) This filter lays the colors out in a grid pattern of alternating colors. Since the human eye is more sensitive to green light, the Bayer mask has two green filters for each red and blue. This creates a truer representation of what the human eye actually sees.

With the color filters, each photodiode can signify a red, green, and blue value. The color value of these cells are combined and interpolated to represent the full spectrum of colors, allowing the image processor to reproduce a full color image.

Currently only the Foveon sensor represents color differently. Instead of using filters, the Foveon sensor places the photodiodes at different depths within the silicon to detect the different colors. This means that each photodiode can detect all three colors without the use of a filter. Although this is proposed to create a truer color reproduction, the Foveon sensor is currently still relatively new and results are currently under scrutiny.

Conclusions

The sensor is one of the most important parts of the digital camera and the size of the sensor is one of its most important aspects. When choosing a digital camera, remember that the compact cameras usually have smaller sensors and the larger SLR cameras usually have larger sensors. Still, the size of the sensor can vary greatly even with the same manufacturer in the same type of camera.

Although smaller sensors allow cameras to be smaller, cheaper, and more portable, when the number of megapixels is very high, a small sensor usually introduces more noise into the photos. Therefore, we recommend that you carefully inspect photos taken with any small point-and-shoot cameras that has more than 6-megapixels to verify that the photos are adequate for your needs. Some small cameras do a good job, like the 10-megapixel Nikon Coolpix S600 that we recently reviewed, but others may not.

Originally at http://www.hardwaresecrets.com/article/Everything-You-Need-to-Know-About-Camera-Sensors/660


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