Spectrophotometer with fan speed control for temperature regulation

In a spectrophotometer having an oscillating grating, an entrance slit and an exit slit and a light source to irradiate said grating through said entrance slit, the grating and the light source are mounted on a heat sink plate and a fan is provided to blow air over both sides of said heat sink plate. A temperature sensing transducer is mounted in the heat sink plate to sense the temperature of the heat sink plate and the output of the temperature sensor is amplified by an amplifier to energize the electric motor of the fan to control the speed of the fan in accordance with the temperature sensed by the temperature sensor to thereby maintain the temperature of the heat sink plate at a constant value upon the system reaching equilibrium. In this manner, the wavelength drift of the system is reduced to a very low value.

BACKGROUND OF THE INVENTION 
This invention relates to spectrophotometric instruments and more 
particularly, to a spectrophotometric instrument provided with a 
temperature regulating system, which minimizes the drift in the wavelength 
and photometric output from the instrument. Spectrophotometers are used to 
measure color and in the near infrared range, to analyze substances. In 
such instruments, a broad band of light irradiates a reflecting 
diffraction grating, which disperses the light into a spectrum. A narrow 
portion of the spectrum is used to analyze the substance. 
The accuracy of the instrument depends upon the capability of the 
instrument to disperse the parts of the spectrum to precisely 
predetermined angular positions in the instrument and with precisely 
predetermined intensities for a given intensity of illumination of the 
grating. Temperature variation in the instrument causes a position of the 
spectrum dispersed from the grating to drift as well as causes a drift in 
the photometric output of the instrument. 
Present state-of-the-art instruments mount the components of the 
instruments on a heat sink, which is cooled by a cooling fan to maintain 
the temperature of the instrument relatively constant. However, such 
state-of-the-art instruments are subject to some drift in the spectrum of 
up to 1/2 of a nanometer in wavelength. To overcome the problem of drift 
in spectrophotometric instruments, it has been proposed to enclose the 
instrument in a cast iron case surrounded by heating blankets, the 
energization of which is controlled by a temperature sensor to attempt to 
maintain the instrument at a constant temperature. Another system has been 
proposed to provide oil passageways in the walls of the instrument and to 
circulate oil through the passageways with the temperature of the oil 
being regulated. Both of the above described systems involve considerable 
structural modification of the instrument and would substantially add to 
the cost of the instrument. 
The present invention achieves temperature control of the instrument by a 
very simple expedient. In the instrument of the present invention the 
components of the instrument are mounted on a thick central heat 
conducting plate, which serves as a heat sink. A fan is provided, which 
directs air over both sides of this plate. In accordance with the 
invention, the speed of the fan is controlled through a continuous speed 
range in accordance with the output signal of a temperature sensor mounted 
to sense the temperature of the heat sink to maintain the temperature of 
the heat sink at a constant temperature. With this system, the temperature 
of the heat sink plate is held at a substantially constant temperature 
about 10 minutes after start up of the instrument and the photometric 
drift is held to a very low value after 10 minutes. The wavelength drift 
in the instrument after the instrument has been turned on for 85 minutes 
drops to less than 0.04 nanometers. Thus, an extraordinary drop in the 
wavelength drift is achieved with a very simple temperature control 
system.

As shown in FIGS. 1 and 2, the instrument is enclosed in a housing 11 and 
has extending vertically through the housing a thick steel plate 13 about 
1/2 inch thick. The plate 13 serves as a heat sink for the instrument. A 
recess 15 is cut in the edge of the plate 13 and in this recess, a fan 19 
is mounted so that a minor portion of the fan 19 extends on the opposite 
side of the plate from that shown in FIG. 1. Accordingly, the fan 19 will 
blow air on both sides of the plate 13. The fan 19 is located adjacent to 
the housing wall 11, which is provided with an aperture closed by a filter 
21 through which the fan 19 draws air to direct it over the plate 13. The 
plate 13 divides the space within the housing 13 into two parts, a smaller 
part on the opposite side of the plate 13 shown in FIG. 1 and a larger 
part on the side of the plate shown in FIG. 1. The space on the side of 
the plate shown in FIG. 1 is divided by a baffle 23, which extends 
diagonally through this space in a zig-zag configuration as shown in FIG. 
1. The space above the baffle 13 is the spectrophotometric chamber in 
which an oscillating reflecting diffraction grating 25 is mounted. The 
grating 25 is oscillated by a motor 26. The oscillating operation of the 
grating 25 is fully described in copending application Ser. No. 
07/294,679, filed Jan. 9, 1989, now U.S. Pat. No. 4,969,739 and assigned 
to the assignee of this application. In a central portion of the baffle 23 
is an entrance slit 27 for the grating 25. An infrared light source in the 
form of a lamp 29 is mounted in the space below the baffle 23 to direct 
near infrared light through the entrance slit 27. A mirror 31 is provided 
to fold the beam of light passing through the entrance slit 27 and direct 
it to the diffraction grating 25. The diffraction grating 25 disperses the 
light in the spectrum, which is reflected toward an exit slit 32 over 
which a cylindrical lens 33 is mounted to direct the light passing through 
the exit slit onto a sample. As the grating 25 is oscillated by the motor 
26, the spectrum is moved across the exit slit 32. At any given time, a 
narrow wavelength band of light from the spectrum will pass through the 
exit slit and be applied to the sample. Because of the oscillation of the 
grating 25, the central wavelength of the band irradiating the sample is 
scanned through the spectrum. 
In accordance with the invention, a temperature sensing transducer 35 is 
mounted on the plate 13, on the opposite side from that shown in FIG. 1 to 
detect and generate a signal proportional to the temperature of the plate 
13. As shown in FIG. 3, the output signal of the temperature sensor 35 is 
applied to a differential amplifier 37, which also receives a reference 
signal voltage from a voltage source 39 applied to the inverting input of 
the differential amplifier. The amplifier 37 amplifies the difference 
between the two signals applied by the reference voltage source 39 and the 
temperature sensor 35 and energizes a DC motor 41 of the fan 19 and drives 
the fan 14 at a speed varying in accordance with the output signal of the 
temperature sensor 35. As the output signal of the temperature sensor 35 
increases, the amplifier 37 increases the voltage applied to the motor 41 
to increase its speed and vice versa. In this manner, the speed of the fan 
motor is controlled to vary directly in accordance with the temperature of 
the heat sink plate 13. As a result of controlling the fan speed in this 
manner, a few minutes after start up, the temperature of the heat sink 
plate is maintained at a substantially constant temperature. 
The graph shown in FIG. 4 is taken from experimental data on a 
spectrophotometer, in which the system of the present invention is 
employed and shows how the output wavelength passing through the exit slit 
32 varies with time after start up for a given angular position of the 
grating 25. The data represented by FIG. 4 was with a warm start-up, which 
means that the electric circuitry of the instrument had been energized, 
with the lamp deenergized, a sufficient time prior to time 0 on the graph 
for the temperature to reach equilibrium. As shown in this Figure, the 
drift drops to and remains below 4/100 of a nanometer 85 minutes after the 
warm start up. Thus, the system of the present invention achieves an 
extraordinary drop in wavelength drift with a very simple system for 
controlling the temperature of the instrument. 
The above description is of a preferred embodiment of the invention and 
modification may be made thereto without departing from the spirit and 
scope of the invention, which is defined in the appended claims.