Parallel, Attenuating Light Rays

This is the simplest correction which is valid only for parallel rays entering the field of view normal to the boundaries.

We assume the intensity of the sheet P decays in the normal manner, viz:

           dP = -A(C) P ds

where A(C) is the attenuation rate as a function of dye concentration, and ds is the incrimental distance a ray of light travels. The observed intensity p is assumed linearly related to the sheet intensity through

           p = F(C) P

where F(C) is the fluorescing efficiency as a function of concentration. We ignore any attenuation of the fluoresced light. For weak concentrations we may take

           A(C) = a C
           F(C) = f C

and combine the expresions for dP and p to show

           dP = - (a/f) p ds = - c p ds.

We may integrate this expression using the known intensity along one boundary to determine the structure of the illuminating field. Finally the concentration may be recovered from

           C = p/(fP)

    NOTE: Before applying this correction, the image intensities should be
    mapped onto a linear intensity scale passing through zero.

Correct LIF image in which buffer ?

This entry specifies the buffer containing the LIF image to be corrected for attenuation in the light sheet.

Store illuminating intensity in which buffer ?

During the correction procedure DigImage determines the spatial structure of the illuminating light sheet. This entry specifies the buffer into which the light sheet is saved. This may not be the same buffer as contains the original image.

Store corrected LIF image in which buffer ?

This entry specifies where the final corrected LIF image is saved. This buffer may be the same as that containing the original image.

Correct in Window or whole Screen ?

The image may be corrected in either a window (<W>) or the entire frame buffer (<S>).

Window Selection: Region to correct for light sheet attenuation

If the user choses to correct for light sheet attenuation in a window, then the Window Selection submenu is produced to aid the window specification process. Additional information on window specification is available through the [H Help] option within the submenu.

Light rays entering flow at Bottom, Top, Left or Right ?

The direction in which the light rays are travelling must be specified in order to calculate the correction. At present DigImage requires the light rays to be (approximately) parallel with the pixel coordinate system, and the intensity uniform at the boundary of the screen or window through which they enter. Care should be used when applying this algorithm to situations where the light rays are not parallel.

Where the light rays enter, assume constant Concentration, constant Intensity or take intensity from Buffer ?

This selection determines the boundary condition on the light rays. The simplest model is that of constant light intensity (<I>). However, often the light sheet is not of uniform intensity where it enters the flow. In some cases the concentration of fluorescent dye will be constant at the entry point (<C>), in which case the entering light intensity may be obtained simply from the image intensity at that point. In the more general case it may be necessary to calibrate the light sheet by using an image of the tank filled with a uniform concentration to obtain an estimate of the intensity of the entering light sheet (<B>).

{If ray start intensities from a buffer}

Ray start intensities obtained from which buffer ?

This prompt is produced only if the ray start intensities are to be obtained from an image stored in a buffer. The entry given here specifies which buffer to use.

Attenuation/fluorescence ratio for dye ?

In order to determine the intensity of the illuminating light sheet, DigImage requires a knowledge of a/f, the ratio of the the rate at which the light sheet is attenuated to the amount of light fluoresced. This is best determined experimentally by considering the light sheet in a region of uniform concentration. This value should then be adjusted until this correction option produces a uniform corrected image.

Scale factor for corrected image (value around 1.0) ?

This entry scales the final corrected image. The higher the value the brigher the final image.

Intensity black offset ?

This value works in combination with the previous entry to rescale the resultant image to maximise the use of the available intensity range.

Message:
Determining intensity of light sheet...

Message:
Correcting for variations in light sheet...

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DigImage User Documentation

Stuart Dalziel, last page update: 19 February 1996