Visible-NIR Spectrometer for Microscope (range 400-1700nm)

New spectrometer design to extend the wavelength range in the near infrared up to 1700nm.

               The small Jobin Yvon H20 monochromator used in the first version cannot be easily modified to change its wavelength range so I have completely redesigned my microscope spectrometer. The new monochromator is a 450mm focal length Cerny Turner design. The optical components are coming from an old Perkin Elmer atomic absorption spectrophotometer. A double grating back to back mounting is used. The first grating is a Perkin Elmer 1440 groves per millimeter, it covers the range 400-1100 nm. The second grating has been bought from Edmund Optics, it is a 600 grooves / mm, it covers the range from 600 nm to more than 2000 nm. The grating can be selected by the computer using an auxiliary motor which turn the assembly by an angle of 180°. The grating position is fined tuned by a stepper motor, computer controlled, actuating the sine bar mechanism.

               The light coming out of the monochromator passes through a home made Nicol prism and is thus polarized. In this design, the Nicol prism replaces the polaroid filters due to the fact that infrared light above 800 nm is no more polarized by conventional microscope polarizer. A first solenoid order filter can be introduced into the light pass. This orange filter rejects the second order of the first grating. The exit lens focuses the light on the microscope stage. A motorized filter wheel can select one of three additional filters into the light pass. A blue filter is used between 400 and 500 nm to reject stray light because the sensitivity in this wavelength region is rather low. A cyan filter is used as an optical attenuator between 500 and 800 nm for the first grating. The third filter is a Kodak 87C filter used as a second order rejection for the 600 gr/mm grating above 900 nm.

              Two different detectors must be used to cover the wide wavelength range. The first one was already described, it uses a silicon photodiode. A second detector has been build with a InGaAs photodiode for the range 600 to 1700 nm. This detector has a 2 stages pre-amplifier with adjustable gain. The power supply is external to both detectors.

              The signal measurement is performed by an Escort digital multimeter and is sent to the computer via a RS232 interface.

              The complete spectrometer is represented  below:

             1: monochromator.  2: Light source enclosed in an aluminum chimney and cooled down by a ventilator.  3: grating motion mechanism actuated by stepper motor not shown.  4: motor used to change the grating actuated by a relay computer controlled.   5: Filter wheel.  6: electronics to control filter wheel, solenoid filter and motor changing  the gratings and power supplies.  7: power supply for detectors pre-amplifiers.  8:  visible detector mounted on the microscope.  9: NIR: detector not mounted.  10: digital multimeter. 11: White light source for microscope.   The red line indicates the light path between monochromator exit and microscope.

2: motor with optical encoder.

3: cyan filter.

4: blue filter

5: infrared filter.