Teledyne Princeton Instruments NIRvana HS, System Manual

Page: 50 / 107

50 NIRvana HS System Manual Issue 1.0
7.3 Noise Sources
Primary noise sources include:
• Dark Charge
NIR signal from the environment and leakage current.
• Input Signal
Photon shot noise.
• Readout
Noise generated when the multiplexers are read out.
An ideal detector contains no noise sources and is limited only by the photon shot
noise. Unfortunately, there are always noise sources that will contribute noise of one
kind or another to the total signal. The methods used to reduce the noise depend on
the noise component being addressed. Infrared noise from the detector and the
environment can be reduced by lowering the temperature of the detector and
environment, respectively.
7.3.1 Dark Charge
Dark charge (also known as dark current) is the thermally induced buildup of charge in
the sensor over time. The statistical noise associated with this charge is known as dark
noise. Dark charge values vary widely from one sensor to another and are temperature
dependent.
With the light into the camera completely blocked, the sensor pixels accumulate
thermally generated electrons, dependent on the exposure time and camera
temperature. The longer the exposure time and the warmer the camera, the less
uniform this background will appear. Thus, to minimize dark-charge effects, the camera
should be operated at its default sensor temperature.
CAUTION!
!
If a sudden change is observed in the baseline signal, there
may be excessive humidity in the camera vacuum enclosure.
Turn off the camera and contact Teledyne Princeton
Instruments Customer Support. Refer to
Contact
Information on page 100 for complete contact information.
7.4 Sensor Temperature
Lowering the temperature of the sensor generally enhances the quality of an acquired
signal by lowering dark noise. In LightField, temperature control is achieved on the
Sensor expander. Initially, the default temperature appropriate for the specific model is
used.
Once the temperature set point has been set, the software controls the camera's
cooling circuits to reach set sensor temperature. Upon reaching that temperature, the
control loop locks to that temperature for stable and reproducible performance. When
temperature lock has been reached (i.e., the temperature is within 0.05°C of the set
value,) the current temperature is Locked. The on-screen indication allows easy
verification of temperature lock.
The time required to achieve lock can vary considerably depending on factors such as
camera type, sensor type, ambient temperature, etc. Once lock occurs, focus may
begin. However, waiting an additional twenty minutes before taking quantitative data is
recommended so that the system has time to achieve optimum thermal stability.