Imaging spectrophotometer

2009/07/03

OK, I have an idea: A spectrophotometer that can take 2D-images, like a normal camera, but from each pixel you get a complete visible spectrum instead of just red, green, and blue intensities.

Pictured is one possible way to construct an imaging spectrophotometer. An image of a subject is focused on a slit by a focusing lens so that only a vertical stripe of the subject will be imaged. A collimation lens aligns the paths of light, so that they will land on a diffraction grating at an ideal angle. Different wavelengths are diffracted to different directions, appearing to the camera to be coming from different horizontal angles. Effectively, the camera images the complete visible spectrum of each pixel on the vertical stripe. The spectra do not overlap as they are spread horizontally on the image. The whole system can be rotated horizontally to scan the scene to form a 2D image.

A drawback with this setup is that the camera and rotation must be synchronized as the camera does not know how much the system has been rotated. This is not so easy to do well, unless the scan is made really slow (not a good thing) or the camera is electronically synchronized to rotation (hard to do and possibly expensive). Perhaps something stationary could be made visible in the lower portion of the image, to act as reference.

It appears this has been done before, for astronomy: http://www.astroman.fsnet.co.uk/needle1.htm

2009/07/08

Masking of stray light was minimal. Also, a reflective slit (a sewing needle) was substituted for the transmissive slit (made of razor blades).

Imaged is a vertical stripe of a fluorescent lamp. The spectrum is shown spread horizontally on the reflective diffraction grating. Because the beams are not collimated, the optimal focusing distance (for the camera) varies by wavelength. Longer wavelengths (red) appear increasingly blurry, as the camera was focused on short (blue) wavelengths. Moreover, the focusing distance that gives the best horizontal resolution deviates increasingly from the vertical one for increasing wavelengths. Such optical anisotropy should also be eliminated by collimation, but at the probable cost of vignetting. Another benefit of collimation is that imperfections of the grating will be out of focus.

For parts, I bought an old Zenit EM film SLR camera from my friend for 20 EUR to obtain both the focusing lens (Helios 44M 58 mm f 1:2) and a fixture for it (the camera body). This lens has some desirable properties: manual focus and manual aperture. The diffraction grating is very large, about 8 cm x 8 cm, scavenged from some sort of a spectrophotometer, at no cost. I'm buying two more of the Zenit cameras, at ebay.co.uk, in hopes that having identical lenses simplifies the design.

2009/07/27

I bought a total of three of these. Each came with a Helios-44M 2/58 kit lens and cost at most 20 EUR a piece, including postage. The razor blade slit will rest on top of the inner film guides (the inner ones of the four bright lines), against which the film normally rests. The outer guides must go as they would otherwise lift the blades off the focusing plane.

Filing the aluminum body was easy. The inner guides were protected by multiple layers of masking tape.

This drastic operation creates some space for the focusing plate from the other camera body.

The camera bodies were mated using two wooden blocks and 8 screws, and a few spacers to correct for any inclination. The important thing is for the two lenses to share the same optical axis, and that they both draw the image on the slit (not yet installed in this picture) when focused to infinity.

Reinforcement bars keep the blades straight. Without them, the blades are too flexible and curve easily, making the effective width of the slit more dependent on viewing angle.

The width of the slit was adjusted manually while the glue was solidifying. Super glue is no good for this as it solidifies instantly.

2009/07/29

Looks promising! The image is quite sharp in both directions, and spectral resolution is awesome. Now need to worry about vignetting (darkening of the image toward corners).

This time using three Helios-44M 2/58 lenses, including one for the Canon EOS 450D camera. There's perhaps something to matched optics. The image has not even been cropped. Also, another diffraction grating is in use. Two diffraction orders are visible, and a bit of ultraviolet light from a third one. The diffraction grating is "blazed" to concentrate transmittance on the shown run of bands. That's why there are no blue bands visible on the left.

Imaged is a circuit board from a dismantled spectrophotometer. :-) The light is from a fluorescent lamp, hence the discontinuous spectrum.

Vertical resolution is satisfyingly good. The slit is perhaps too wide; some horizontal features of the imaged object are visible (not at the low resolution of this web image), especially at the narrow ultraviolet band. But this is good enough for now.

2009/07/30

The 1200 grooves/mm grating gives better spectral resolution than the 300 grooves/mm grating in the earlier picture. But also, there is not much headroom for wavelengths outside red and violet in case I get a camera that can capture a wider range of wavelengths than is visible to humans. It is possible to modify the Canon EOS 450D by removing a filter inside it, but I'm a bit scared of the process. I won't consider it before this thing is working.

These kind of gratings cost around 100 EUR a piece new, but mine were scavenged from spectrophotometers thrown away by the chemistry department, so, free.

The continuous stripe of spectrum is from the flame of a Colt cigarette lighter. :-) The background is the usual circuit board illuminated by a fluorescent lamp. There's some ghosting in the upper left corner, probably because of reflections inside the lenses. The flame was quite bright.

What the objective lens drew on the slit can later be recomposed from the spectrum. The process is illustrated. All wavelengths are simply squeezed back together. (It looks black and white, but it's the fault of jpeg image compression)

2009/09/11

Before operation, the optics are covered with pieces of black leather.

The resulting image of the first manual scanning test. The actual scan:

The video was recorded while the system was manually rotated counterclockwise. So in fact this video encodes a complete image! Points to anyone who can tell what the scene is (Joonas got some points for this!). The original resolution of each video frame from Canon EOS 450D was 848 x 560. The final vertical resolution is thus 560 pixels. With a more coarse diffraction grating, it would be possible to rotate the camera 90 degrees to get a vertical resolution of 848 pixels. The borders are just for contrast.

The image file encoding gamma 1/2.2 was taken into account. Basically, before doing arithmetic on pixel RGB values as if they were proportional to luminous intensity, they were raised to power 2.2. And before the results were written back into an image file, the inverse operation, power 1/2.2, was employed.

Still tons of things to improve in how the image is generated, but it's a good start. List of things to do:

• Correct optical distortion by postprocessing to reduce color bleeding.
• Figure out where each wavelength is.
• Take into account Canon EOS 450D RGB sensor sensitivity curves to improve dynamic range of pixels that have some of the color channels saturated.
• Replace the Canon sensor RGB sensitivity curves with CIE 1931 XYZ color matching functions to better model human color vision. (Canon ain't perfect!)

2009/08/12

A lot of extra dynamic range could be mined from this. But it needs a bit of detective work. The color space is sRGB, which is not directly the raw sensor data, but something like a linear combination including negative coefficients.

2009/08/13

Fraunhofer lines can be used for wavelength calibration. This image is for demonstration, not an actual calibration. Sandwiched is the measured spectrum of the evening sun, cut in two to increase detail in the web image. Some major and some minor Fraunhofer lines are visible.

• G, 430.8 nm, iron and calcium
• F, 486.1 nm, helium
• b4, 516.9 nm, iron
• E, 527.0 nm, iron
• D3, 587.6 nm, helium
• a, 627.7 nm, atmospheric oxygen
• C, 656.3 nm, hydrogen

There are many additional lines visible, some stronger than some of the designated ones.

Fraunhofer lines are due to absorption of light by elements in sun's outer layer and by oxygen in Earth's atmosphere.

2009/08/16

The spectrum image geometry was corrected by manually adjusting a 3x3 grid of control points and by biquadratic interpolation of the coordinates of the control points in the source image. The basis functions used were:

(0.25*x^2 -     x + 1)    * y[0] +
(-0.5*x^2 +     x)        * y[1] +
(0.25*x^2)                * y[2]

Bicubic Catmull-Rom interpolation (at gamma 1.0, of course) was used for reading the source spectrum image.

For the calibration, imaged was a black metal grid against sunlight. This provided both horizontal (metal grid) and vertical (Fraunhofer) lines that were then made straight by adjusting the control points.

The bottom row shows the resulting enhancement for the first manual scan test. Vertical color fringing is reduced.

Some types of distortion cannot be corrected by a 3x3 grid of control points, for example bulging that makes the image more dense toward the edges. So, I derived piece-wise quadratic basis functions for 4 control points. I might implement them later. Here goes:

Left block, x = 0..1:

(0.25*x^2 - x + 1) * y[0] + (-0.5*x^2 + x) * y[1] + 0.25*x^2 * y[2]

Middle block, x = 0..1 (for convenience, instead of x = 1..2):

(0.25*x^2 - 0.5*x + 0.25) * y[0] + (-0.25*x^2 + 0.5) * y[1] + (-0.25*x^2 + 0.5*x + 0.25) * y[2] + 0.25*x^2 * y[3]

Right block, x = 0..1 (for convenience, instead of x = 3..4):

(0.25*x^2 - 0.5*x + 0.25) * y[1] + (-0.5*x^2 + 0.5) * y[2] + (0.25*x^2 + 0.5*x + 0.25) * y[3]