Spectral Cameras Support and Resources

What Is Multispectral Imaging?

Spectral imaging is a combination of imaging and spectroscopy, where a complete spectrum is collected at every location of an image plane. This powerful technique is sometimes called hyperspectral or multispectral imaging. Spectral imaging is not restricted to visible light, but works from ultraviolet to infrared. Wikipedia offers a very good overview of hyperspectral imaging: http://en.wikipedia.org/wiki/Hyperspectral_imaging

Spectroscopy captures the entire spectrum, light intensity as a function of wavelength. It's this very detailed spectral response curve that gives spectral imaging the ability to discriminate specific chemicals and elements. The unique reflections and absorbances are the signature of the compound.

Frequently Asked Questions

There’s not enough light in my image
My image is out of focus
Low S/N at the short (blue) wavelengths
Low S/N at the long (red/infrared) wavelengths (V10E)
Wrong or shifted spectral region
Vertical stripping in my image
Interference fringes in my image
I don’t see an image (software)
I don’t see an image (hardware)
What is an imaging spectrograph?
What is a spectral frame?
How is the number of pixels related to the spectral resolution?
How do I image a target with an imaging spectograph?
What is a data cube?
What is a waterfall image?
How fast can I image a target?
Why is my image not correctly proportioned when I display a waterfall image?

There’s not enough light in my image

Is the lens cap on the objective lens?
Is the lens aperture open?
Do you have adequate integration time?
Do you have an incompatible light source ie is there an IR cutoff filter?
The target has high absorption/low reflectance.

My image is out of focus

The back focal length of the lens is incorrect for the camera (may not be C-mount).
The lens is not focused on your target. Use a focus target to set the focus. See the technical note Focusing and alignment (pdf)
The objective lens is loose or incorrectly installed.
The objective lens is not suited for spectral imaging (low quality, wrong wavelength range, coatings).

Low S/N at the short (blue) wavelengths

The light source (usually halogen) does not produce much energy at the short wavelengths.
The camera detector has low response at the short wavelengths.
There is a lens coating on the front objective or a UV blocking filter present.

Low S/N at the long (red) wavelengths (V10E)

The light source has an infrared cutoff filter or the fiber optics absorb the light.
The camera is equipped with an infrared cut-off filter (hot mirror).
The detector has low response (low QE) above 700 nm.
The front objective coatings are not designed for above 700 nm.

Wrong or shifted spectral region

Incorrect calibration – the spectral lines from a reference source have not been correctly identified. You can use a simple fluorescent table lamp to indentify spectral lines.
The camera detector is too small, misaligned or not centered.
There are calculation errors.

Vertical stripping in my image

There is likely dust on the entrance slit of the imaging spectrograph.

Interference fringes in my image

Some CCD and CMOS detectors have a thin coating on the detector surface causing interference phenomena (like Newton rings) that are seen as horizontal waves. This is an aesthetic problem only and does not interfere with spectral imaging.

Interference Fringes

I don’t see an image (software)

Check your configuration software (such as MAX or Device Manager).
You are using the wrong camera drivers.
You are missing DLLs.

I don’t see an image (hardware)

Check your data cable.
Make sure the lens cap is removed.
Is the power supply connected?

What is an Image spectograph?

An imaging spectrograph transforms a very thin slice of an image into its spectral components by using a prism, grating or both and projects the spectral information onto an imaging sensor, typically a scientific CCD or CMOS camera.

What is a spectral frame?

A spectral frame is the image captured by the imaging spectrograph. The horizontal dimension or row is spatial. The field of view is defined by the focal length of the objective lens, distance to the target and width of the sensor. This field of view is then divided into the number of pixels of horizontal resolution. The vertical dimension is spectral. Each column of pixels placed on the sensor represents the intensity of light reflected from a portion of the thin slice of the target at a particular wavelength.

How is the number of pixels related to the spectral resolution?

The spectral resolution of the imaging spectrograph is defined by the optics of the prism or grating mechanism and the entrance slit width of the device. The light entering the system is defracted into its components according to wavelength. For example Specim’s ImSpector V10-e provides a spectral resolution

How do I image a target with an imaging spectograph?

A target is imaged by first determining the height of the scene that the spectral imaging system is exposed to. This is determined by the focal length of the lens, the imaging slit width and the distance to the target. If our height works out to be 0.5 mm, we must take a spectral frame, move either the imaging spectrometer or the target 0.5mm then take the next spectral frame, repeating this process until the entire scene has been imaged. If we move less than 0.5mm we will be oversampling the scene, repeating the data gathered from a single point. If we move further than 0.5mm we will be undersampling, missing data from the target we are imaging.

What is a data cube?

A data cube is simply a collection of sequential spectral frames placed back to back.If we imaged a target with our 1024 pixel x 1024 pixel imaging spectrograph using an imaging height of 0.5mm and took 200 images the dimensions of our cube would be frames x pixel width x pixel height or 200x 1024x1024.

What is a waterfall image?

A waterfall image is simply a spatial image taken from our data cube. If we take an slice from the data cube in theframe x pixel width (the two spatial dimensions) we will get a recognizable image of the target at a particular wavelength.

How fast can I image a target?

This is a simple question with a complicated answer. The imaging speed is determined by:

·The sensitivity of the camera and the illumination of the target (lower sensitivity or lower light levels require longer integration times)

·The data transfer capabilities of the camera

·The pixel depth (an 8 bit pixel is ½ of the data of a 10 or 12 bit pixel)

·The transfer speed of the camera to computer interface (CameraLink is fast, USB is much slower)

·The computer’s ability to process the incoming data

A fast camera with lots of light can produce more than 100 full spectral frames per second.This means if the imaging height of the scene is 0.5 mm you can image more than 50mm/sec. Handling the data at the computer becomes a problem as this 100 frames/second results in 50 megabytes of data per second that needs to be processed. A number of compromises can be made including decreasing the bit depth or the spectral or spatial resolutions depending on the application.

Why is my image not correctly proportioned when I display a waterfall image?

When a spectral camera images a scene, the frame can be considered to be three dimensional. What the viewer sees when viewing the image is the two dimensional spectral frame which is defined by the area of the detector. This frame typically has data for each pixel of the camera.What must be remembered is that this is the spectral image of an area defined by the optics of the spectral camera. If the scene being imaged is 0.5mm as an example, each pixel can be considered a 3d cube defined as pixel height x pixel width x scene height. If the scene height and the pixel width are not equal, a waterfall image which is simply a slice taken through the data cube will present a rectangular pixel defined as scene height x pixel width. When this image is presented on a screen with square pixels, the image will appear to be “compressed”, even though the data is completely valid.