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Image Sensor Sizes

There are at least three methods by which image sensor size categories are assigned.  Since two of these methods produce category labels that look almost the same, confusion is common.  Here, we try to sort this out.

Method 1 - Referenced to Image Tube Sizes

When the vidicon tube was invented in the early 1950s, the glass tubing used to make the first units had a nominal exterior diameter of one inch.  These were referred to as "One-inch tubes".  After allowing for the space taken up by the interior metal structures needed to control the electron beam, the portion of the photoconductor on the front glass accessible for scanning had a diameter of about 5/8 inch.  Early television used a raster aspect ratio of 4:3 so the corresponding scanned area was 1/2 inch horizontal by 3/8 inch vertical - the canonic 3:4:5 right triangle.  After a while, the image size assumed the name of the tube and a 5/8 inch diagonal raster became commonly called the 1-inch size.  5/8 inch is 15.875 mm and so 16 mm became the designated metric diagonal for 1-inch tubes.

Later, three more glass sizes became common; 2/3-inch and 1/2-inch for lower-cost cameras for scuurity and compact, rugged cameras for defense and 30 mm (converted in the reverse to 1.2 inches) for early Philips Plumbicon tubes, which needed some extra diameter to achieve resolution acceptable for broadcast television.  The corresponding raster diagonals for these glass sizes were 11 mm, 8 mm and about 21.4 mm.

From these few numbers, all of the smaller imager sizes (and one larger size) were scaled.  In some cases the scaling was approximate, partly because 11/16 is not quite 2/3 and partly because other sensor sizes were built and then squeezed into existing size categories.  Now, there are very many actual raster (active area) sizes with small variations from the calculated nominals but the categories remain useful to group sensors for comparison.  Table 1 below shows raster sizes commonly included in the size categories.  Although most of the early imagers had a 4:3 aspect ratio, now sensors are found with 16:9 AR (HDTV), 3:2 AR (digital still camera) and 5:4 AR (computer graphics).  There are also may special-purpose sensors with other aspect ratios.  All of these are still classified by their image diagonal because that is the key parameter needed to assure proper coverage by lenses. 

Table 1 - Dimensions Inch-Equivalent Sensors with 4:3 Aspect Ratio

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For comparison, Table 2 shows dimensions for 2/3" sensors with other aspect ratios.

Table 2 - Raster Sizes for 2/3" Sensors of Various Aspect Ratios

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Method 2 - Referenced to a 35 mm Film Frame Size

When lenses are used on digital cameras for photography, the users find it convenient to think of the camera field by considering what focal length lens is required with the small sensor in the camera to give the same field of view that they used to get with a 24 x 36 mm film frame.  This focal length ratio is expressed in terms of a the focal length multiplier, the ratio of the size of the sensor in the camera in use to the 24 x 36 mm frame.  If the sensor is 16 x 24 mm, for instance, the size ratio is 1/1.5 so to get the same field of view with the digital camera as the 35 mm film camer would have had, the lens focal length must be reduced by 1/1.5.

For convenience to the user, the sensor iteslf is also referred to as having a 1/1.5 size.  This can cause confusion for engineers looking for sensors because for photographers, a 1/1.5 sensor is 16 x 20 mm but in video camera use a 1/1.5 sensor with a 3:2 aspect ratio is a 2/3" inch sensor with a raster size of 9.2 x 6.1 mm.  This is not a mistake that anyone wants to make.  Table 3 shows some common sensors sizes (all 3:2 aspect ratio) that are disugnated by the focal length multiplier.

Table 3 - Raster Sizes for Various Focal Length Multipliers

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Method 3 - Referenced by Name

Some sensor sizes correspond to specific sizes used elsewhere.  These sizes identified by the names of these references.  Table 4 gives a few examples.

Table 4 - Raster Sizes Referenced by Name

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Well, this is all really interesting and useful as a set of rules of thumb, but always check the actual size of any sensor you intend to use because the designators are often loosely applied.