Intro to IR (Part 5): LensMay 07, 2018
Because glass blocks electromagnetic radiation of infrared wavelength, so that it doesn’t reach the camera’s detector, lens for IR cameras are made from different materials that have high refractive index and low optical dispersion.
Germanium is a common choice, optically best suited to handle photons radiation as it is the most efficient available material to transmit energy to the detector. It is widely used for uncooled IR cameras where it’s covered with an engineered coating. Germanium lens appears to be black to human eye.
There are many other materials like calcium fluoride, fused silica, magnesium fluoride, N-BK7, potassium bromide, silicon, sodium chloride – each with their own unique attributes that make them suitable for specific applications.
It is important to know what type of lens the camera uses and understand the different applications each type is good for.
The two most common specialty lenses on the market are fixed-zoom (aka manual focus or multifield) lens and continuous-zoom lens. The fixed-zoom lens can switch between two or more focal lengths, while the continuous-zoom one can remain in focus anywhere between two boundary focal lengths. These lenses can survey a scene and magnify an object within that scene switched or zoomed respectively. For example, a 50/250 dual-field-of-view lens can be switched from a 50-mm focal length to a 250-mm one to yield a 5X magnification of the object being viewed.
Below are the most common lens types and their features. Each type is more suitable for a certain application:
- Fixed-zoom: Zoom is fixed (several focal length are available). Fixed-zoom lens is sensitive to environmental temperature changes and this calls for focus readjustment; useful for indoor systems (acclimatized areas) or for handheld cameras when the operator has free access to the lens.
- Continuous optical zoom: Enables zoom and focus remote control; maintaining focus during zoom adjustment; auto focus.
- Athermal manual fixed focus: Manual focus enables adjustments to customer needs; Includes compensation mechanism for temperature changes; ideal for outdoor applications.
- Motorized focus: Ideal when scanning near and distant targets; features fixed zoom; internal motor adjusts focal length; allows for manual remote control of the focus.
- Dual FOV: FOV can be switched remotely as needed; focus is automatically adjusted by an additional motor.
Worth noting here, many lenses are designed to operate in a specific waveband. For example, 1.5 to 5 or 3 to 12 µm. When designed for a particular waveband, they will underperform when used for imaging in other wavelengths. So choose a lens that are specifically designed to perform in the range necessary for your application to ensure best results.
Detector’s pixels sensitivity is not uniform. During longer exposure shots certain pixels will give brighter intensities. In other words, different pixels may produce different signals of the same scene. This problem may arise from tiny variations in pixel sizes, materials, or interference with the local circuitry, and by changes in the environment like temperature fluctuations. Also, the detector’s pixels can start “drifting,” as the camera and the detector heat up during use.
The non-uniformity correction feature (NUC) of a camera counteracts that. It enables the camera’s core to calibrate the uniformity of the detection array. The procedure is run digitally and mechanically and includes viewing a uniform IR source, comparing the sensors/pixels outputs, and compensating for non-uniform sensors.
Three common NUC methods are:
- Using a shutter – when a controlled shutter blocks the core detector view for a moment while the signal is processed
- Using a lens cover – when the lens view is blocked
- By “de-focusing” – when motorized lens can de-focus to calibrate the image.
Remember to choose a camera that is able to effectively perform non-uniformity correction, as such camera will produce a superior quality image. Also, confirm that the camera performs NUC periodically and more frequently when it first starts.
In this series, we have covered many factors that must be considered when choosing a thermal imaging camera and a right set of lens for it. We hope knowledge of the terminology and technology used by camera manufacturers will now arm you to choose the best thermal camera for your needs.
Read the previous posts in the series:
Part 1 – The physics behind thermal imaging
Part 2 – Cooled vs. uncooled cameras, sensitivity, resolution, frame rate
Part 3 – Sensitivity, resolution and frame rate
Part 4 – Optics