Intro to IR (Part 4): OpticsApril 30, 2018
Have you been puzzled by all the optics terms you saw in camera specifications or all the different types of lenses available?
Because most people have little knowledge of optics and lens design, it can be difficult to judge lens performance outside of a subjective visual evaluation. The most accurate way is to dig into the numbers and compare the specifications. Below, you’ll find some of the most common optics terminology used in specifications and descriptions of different camera and lenses.
Focus, focal length, and focal point
When judging the performance of a thermal camera, the ability of a camera to focus on an object is of fundamental importance. Because changes in the ambient temperature can result in the camera’s degraded focus, it’s important that the camera is able to compensate or counteract such fluctuations. A well-focused image is essential for effective detection and analytics.
One common specification which determines lens focus is focal length. Focal length is the distance between the optics and the focal point (point of focus). Object will be in focus when the distance between the optics and the sensor is the same as the focal length.
Manufacturers have different lens classes based on the focal lengths. Common longer focal length lenses are as follows:
- Telephoto – the lens physical size is shorter than focal length;
- Normal – an image created by the lens is close to what the human eye can see;
- Wide angle – the lens produces a scene wider than normal vision; the lens with a field of view greater than 150° are called “fish-eye.”
Field of view
Another specification you will come across is field of view (FOV). FOV is the visible extent of a scene captured by the image sensor and it is directly correlated to the lens’s focal length (and detector area).
As the focal length increases, the field of view for that lens will be narrower because the view angle gets smaller, and magnification gets bigger. Conversely, as the focal length decreases, the field of view widens and magnification capabilities get smaller.
It is important to familiarize with each lens’ horizontal and vertical field of view. Companies will often use this to specify lenses for a particular model. The horizontal number defines the angle that spans the width of the focal plane array, and the vertical field of view defines the angle spanning the height of the array.
F-number and focal ratio
F-number (f/N or f#) is the ratio of imager focal length to diameter of the front lens. Is it the amount of light – or in the case of IR imaging, the amount of energy – that reaches the detector.
Ultimately, this number tells you how sharp the image will be. For the same focal length, the lower F#, the larger the optics will be and more light/energy reaches the detector, and more of that means more detailed image.
Higher F# means high local length lenses are smaller in size which makes the device more compact. And more affordable, too, as the main cost driver of these designs is the lens’s diameter. At the same time, the lens must be sufficiently large to avoid vignetting of ray bundles intended to reach the detector.
Generally, a lower F# is preferred for the sharpest image. But, depending on the application, this could be an opportunity to make trade-offs to minimize lens size and weight.
The f number is also referred to as the speed of the lens. For example, an f/2.3 lens may be considered faster than an f/4.0 lens. To give an example, best uncooled cameras lenses are F# 1.0-1.4. Cooled cameras can have high (4-5) F# lens because their detector is more sensitive.