Grating ozone spectrophotometer

A spectrophotometer having means for automatically linearising the output signal of a photomultiplier tube comprising:

• a housing having mirror means for collimating incident light input, diffraction grating means for diffracting the collimated light to give a light spectrum,
• a plurality of exit slit means located in the focal plane of the spectrophotometer, each exit slit means corresponding to a particular wavelength of the light spectrum, said photomultiplier tube receiving focussed light from said exit slit means and converting said focussed light into electrical signals,
• wavelength cycling means located in the housing, said wavelength cycling means having a mask for covering said exit slits with a plurality of apertures therein, corresponding to respective exit slits,
• said mask being movable to bring a respective aperture into register with a respective exit slit to allow light at that particular wavelength of pass to the photomultiplier tube,
• said mask having at least two additional apertures for permitting light from two particular wavelengths to pass to said photomultiplier tube at the same time, and
• control and computing means for calculating the sum of the light intensity from two wavelengths at different times, and comparing said sum with the value of light intensity obtained in response to light from two particular wavelengths being passed to said photomultiplier tube at the same time, said control means constituting means effective to use said comparison to automatically linearise said photomultiplier tube output.

A method of determining the ozone and sulphur dioxide gas concentration in the atmosphere, comprising a first mode, said first mode including the steps of:

• observing the atmosphere and receiving incident light therefrom,
• diffracting said incident light to produce a diffracted signal having a frequency spectrum with wavelengths a function of the said gases in the atmosphere,
• passing said wavelengths through selected slits in a spectrophotometer focal plane to provide light of particular wavelengths, detecting the intensity of said particular wavelengths using a photomultiplier tube and generating corresponding electrical signals therefrom,
• said method having a second mode exclusive to said first mode and including the steps of:
• providing an additional slit and a calibration light source for causing light to pass through said additional slit,
• scanning said light which passes through said calibration slit and producing a signal representative of the median wavelength intensity of said calibration light,
• controlling the location of said diffraction grating using said median wavelength signal to adjust said grating to maximise the intensity of said median wavelength signal to provide wavelengths of a predetermined value at said respective slits.

A method of automatically linearising a spectrophotometer output signal comprising the steps of:

• recording the light intensity of a first wavelength at a first time,
• recording the light intensity at a second wavelength at a second time,
• adding the light intensities recorded at said first and second wavelengths to give a first summed intensity value,
• recording the light intensities from both first and second wavelengths at the same time to give a second summed intensity value,
• comparing the sum of said first and second summed values,
• processing the difference between said first and second summed values to provide an indication of deadtime, and using said indication of deadtime to compensate for non-linearity in the spectrophotometer output signal.