The monochromator initially connected to the microscope has been modified to make NIR spectroscopy up to 2550 nm. The spectrometer part remains the same with two gratings: one 1440 lines per mm blazed at 540 nm for the visible part of the spectrum and the second one for the NIR range with 600 grooves per mm blazed at 1000 nm. The Nicol polarizer has been removed. The filter wheel was rebuilt as described below and a box with optics and detectors has been added to the system.

             Above is a photograph of the detector and accessories box. The light coming from the monochromator is reflected at 45°, passes through a lens to be focused on the detector if transmission mode is used or focused on the sample if reflectance mode is used. This detector box is very flexible. The optical arrangement can be quickly modified from transmission to reflection, the detector can be changed from a visible range photodiode to NIR extended range photodiode. A PbS photoconductive cell can also be used. This cell requires an optical chopper described below. The motor of this chopper is visible in the picture above. The image above illustrates the transmission mode. The gray filter is removed for reflection measurement, the photodiode detector is turned to the back of the box, the transmission cell holder is removed and replaced by a reflection holder.

             A PbS photoresistor is normally used with an AC amplifier to reduce the noise. The light beam from the monochromator is modulated by a chopper. My PbS cell is a Thorlabs component.  In the specification of this component, it was advised to use an amplifier  at 600 Hz. In fact, the frequency response of the PbS device is so low that I had to change my first design and lower the frequency to 7 Hz otherwise the sensitivity of the detector was much to low. Above is the design of the amplifier, a two stages amp working at about 7 to 10 Hz. The trans-impedance amplifiers used with the photodiodes cells have been described elsewhere.

            The AC amplified signal from the PbS resistor must be rectified before it can be sent to the analog to digital converter. The circuit above is a full wave rectifier with a low pass output filter. Below is a photograph of the detectors amplifiers section: on the left the photodiodes amps with the AD549 metal cans op amp and on the right the PbS amplifier with the rectifiers. The PbS cell is polarized by three 9V batteries connected in series to get 28.5V.

              The chopper for the PbS detection has been manufactured with a CD where 8 holes have been drilled to pass the light.

                    New filter wheel with 8 filter positions actuated by a stepper motor. A blue filter is used in the range 400 to 480 nm to reduce the stray light, an orange filter is introduced above 600 nm to remove the grating's second order, another filter removes the second order above 840 nm. In the NIR, I'm using the 880 nm and a 1400 nm filter as second order removal. Some gray filters are also added to the wheel to attenuate some parts of the spectrum. A solenoid gray filter is removed from the beam for wavelengths above 2200 nm as the grating intensity is much lower.

                All the devices of the spectrometer are controlled by an Arduino mega 2560. It has two stepper motor controllers for the scan of the wavelengths and the rotation of the filter wheel. It also actuates the relays for the grating selection and both solenoid filters motions. There is an input for the 3 detectors whose signals are fed into a 4 channels  24 bits ADC. The Arduino program controls the whole system, it receives the commands and send the data to a PC. An Excel program has been developed as a user interface.

          View of the 3 detectors. The photodiodes can be placed very close to the illuminated area of the sample in reflection mode to increase the signal. In transmission mode, the beam out of the monochromator is focused onto the detector.