Digital Camera & Thermal Imaging Sensor Sizes Explained

Infiniti Whitepaper

Let’s face it, digital camera sensors are measured in ridiculous ways. If you’re trying to make sense of what dimension a 1/2.8" sensor size is referring to, or if you’re wondering how thermal imaging sensor measurements are different, you’ve come to the right place.

Visible Imaging Sensor (RGB Color Camera)

An imager that collects visible light (400 to 700nm) and converts it to a color image that replicates normal human vision.

Micron (µm)

Microns, also known as micrometers (represented as µm) are a length of measurement equal to one millionth of a meter. (1,000µm is equal to 1mm.)

Pixel Pitch

Pixel pitch is the distance between two adjacent pixels in the sensor, measured from pixel center to pixel center.

Glossary

35mm Sensors

First let’s get the well-known 35mm format out of the way. The exact dimension of a “35mm sensor” measures 36×24mm, which doesn’t seem to have any logical relation. That’s because the name is in reference to 35mm film, which was the most common film size before digital cameras came around. It was named because it was 35mm in width, however the sprockets and information that needed to be included on the film resulted in an image size that was roughly 36×24mm. Obviously, digital sensors don’t need the sprockets and frame information, but the name stuck so that DSLR manufacturers could market their cameras as having the same size sensor as their traditional film cameras. (Thus you could use the same lenses and expect the same field-of-view.)

Inch Fractions

Other digital camera sensors get their size names and numbers from a much more bizarre history. When video cameras were invented back in the 1950s, they used vacuum tubes to capture the image, much like televisions of the day used vacuum tubes to display the image. These sensors were measured by the outside diameter of the tube, which typically produced a usable image about 2/3 of that size.

When smaller digital sensors were developed, they decided to measure them based on what the tube size would be if it were a 1950s video camera. As a result, camera sensors that measure themselves this way (1", 1/3", 1/2.8", etc.) typically have a diagonal measurement roughly two thirds of that diameter. However, to make matters even worse, manufacturers often wanted the sensor size to be a simpler fraction, so instead of being a more exact 9/16" or 27/32" for example, measurements were often rounded up or down to the nearest simpler fraction. This means it’s possible to have multiple digital sensors that are all classified as 1/3" sensors but with different actual dimensions.

As more smaller digital sensors needed more differentiation, decimals were added for the measurements between 1/3" and 1/2", and between 1/2" and 1". We now have sensors categorized into various measurements involving fractions with decimals (1/1.9", 1/2.8", etc.), but all are still rough approximations of what the diagonal tube size might be. To make matters even worse, the practice of measuring sensors based on their diagonal measurement means that sensors with a widescreen aspect ratio fall into a different size category than a sensor that's the same width but in a 4:3 aspect ratio. Meaning a smaller widescreen sensor can offer a wider field of view than a larger “fullscreen” sensor.

Pixel Pitch (How Thermal Imaging Sensors are Measured)

If you dig deep into the technical specifications of a digital image sensor, you’ll eventually find a measurement for the “pixel pitch” of the sensor. The pixel pitch is a more precise and straightforward measurement that unfortunately isn’t mentioned often in standard camera specifications. Thermal imaging sensors on the other hand (MWIR and LWIR), as well as SWIR sensors, are all measured by their pixel pitch.

The pixel pitch is quite simply the width of a pixel on the sensor, typically measured in microns (μm). This makes things simpler for calculating the overall physical sensor size, simply multiply the resolution by the pixel pitch. Unfortunately it creates confusion when it comes to how those thermal or SWIR sensor sizes affect the field of view and detail levels of the camera.

With standard visible sensors, increasing the resolution without changing the sensor size increases the amount of detail in the image, however with thermal cameras increasing the resolution while maintaining the pixel pitch gives the same amount of detail on an image but with a wider field of view.

Infiniti accounts for these differences by measuring the camera’s effective detail using PPM. We also use an internally developed tool that can preview the field of view for most sensor sizes at any focal length. Please contact us today for help with selecting the best camera for your application and budget.

Illustrated comparison of how physical sensor sizes change when sensor resolution doubles (visible vs thermal).