External coupling optics, including a spherical mirror, is located in the beam path between a curved slit and a detector for imaging the slit onto the detector; alternatively, the beam path is between a light source and the slit for imaging the slit onto the source; alternatively, the beam path is between the slit and a sample cell for imaging the slit onto the cell.

At least one of the entrance slit and the exit slit of a monochromator is capable of being opened or closed. The light issued from the exit slit when the slit width is changed is detected by a light detector. The slit width is calibrated when the ratio between the maximum value of the output of the light detector and the output of the light detector with the slit being gradually closed subsequently reaches a predetermined value, in other words, when a predetermined narrowness of the slit is attained. The slit width is controlled by a pulse motor or the like. The pulses supplied to the pulse motor are counted by a counter. At the time of slit width calibration, a value corresponding to the particular slit width is set in the counter, thus performing the slit width calibration.

A spectrometer assembly (10) is disclosed. The assembly includes a light source (11) with a continuous spectrum. A pre-monochromator (2) generates a spectrum with a relatively small linear dispersion from which a spectral portion is selectable, the spectral bandwidth of the spectral portion being smaller than or equal to the bandwidth of the free spectral range of the order in the echelle spectrum. The centre wavelength of the selected spectral interval is measurable with maximum blaze efficiency. The assembly also includes an echelle spectrometer (4) with means for wavelength calibration, an entrance slit (21) at the pre-monochromator (2), an intermediate slit assembly (50) with an intermediate slit (3) and a spatially resolving light detector (5) in the exit plane of the spectrometer for the detection of wavelength spectra.

A fluorescence spectrophotometer having an excitation double monochromator, a coaxial excitation/emission light transfer module, and an emission double monochromator. Each monochromator includes a pair of holographic concave gratings mounted to precisely select a desired band of wavelengths from incoming broadband light without using other optical elements, such as mirrors. Selected excitation light is directed into a sample well by a light transfer module that includes a coaxial excitation mirror positioned to direct excitation light directly to the bottom of a well of a multi-well plate. Fluorescence emission light that exits the well opening is collected by a relatively large coaxial emission mirror. The collected emission light is wavelength selected by the emission double monochromator. Selected emission light is detected by a photodetector module.