Analog Mode
The OBB Corp. Photometers use analog form of detection, which is suitable for medium to high levels of intensity. It is the common form of detection for microscopy. The other level type of detection is photon counting which is the best for low light levels. In this instance you may want to contact www.pti-nj.com who are probably exclusive in this type of detection for microscopes.
The major reason why one uses analog over photon counting (digital) detection wherever possible is cost and more dynamic range Analog detection however is more noisy and hence does not detect low light levels well. Fortunately for most measurements in microscopy analog detection is sufficiently sensitive.
How it works:
In analog detection, the pulses from the photocathode are effectively averaged by the high impedance at the anode and the stray capacitance. The resultant photocurrent is converted to a voltage that is proportional to the intensity of the light striking the photocathode of the tube. The maximum PMT current is 10 microamperes. The output is a DC signal between +13 volts and -13 volts. The high voltage driving the tube may be varied to change the sensitivity of the PMT. The time-constant on the PMT may be adjusted to control the noise levels of the measurements. The analog output can be displayed on the LED screen of the PMT. The signal from the PMT is digitized by an A/D Converter on an I/O Board and stored in a computer for subsequent analysis. One can select for the Model 4300/4301, to display the anode current of the PMT.
The Detector—Photomultiplier Tubes
OBB’s PMT housing will accept a wide variety of side-on 1 1/8-inch diameter photomultiplier tubes. They are optimized for PMTs with uniform dynode chains. PMTs are normally selected based on their wavelength response and their noise and/or sensitivity characteristics. OBB offers two different PMTs, as standard, for use with our housings. We can and do offer special PMTs for custom applications.
Specifications |
302 PMT |
301 PMT |
| Spectral Response |
185 to 680 nm |
185 to 900 nm |
| Wavelength of Maximum Response |
400 nm |
400 nm |
Cathode Sensitivity
Luminous
Radiant at 400 nm |
60 µA/lm
60 mA/W |
250 µA/lm
74mA/W |
Anode Sensitivity (at 1000 V)
Luminous
Radiant at 400 nm |
400A/lm
4.0 x 105 A/W |
2500 A/lm
7.4 x 105 A/W |
| Low Dark Current |
0.1 nA |
3 nA |
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302 PMT
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301 PMT
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How to Choose
The major difference between the two PMTs is spectral range of detection. However unless you need to detect in the extended NIR range (680–900 nm), we would recommend the 302 PMT. It is less noisy and is about half the price.
How it works
A PMT consists of a light-sensitive photocathode that generates electrons when exposed to light. These electrons are directed onto a charged electrode called a dynode. The collision of the electrons with the dynode produces additional electrons. Since each electron that strikes the dynode causes several electrons to be emitted, there is a multiplication effect. After further amplification by (typically) nine dynodes, the electrons are collected at the anode of the PMT and output as a current. The current is directly proportional to the intensity of light striking the photocathode.
Because of the multiplicative effect of the dynode chain, the PMT is a virtually noiseless amplifier with a gain of about a million. The result is very high sensitivity and remarkably low noise. For this reason it is virtually impossible to beat a PMT for sensitivity in the UV-VIS region.
PMTs are also quite robust. When properly cared for, a PMT will typically function for 10,000–100,000 hours.
Adjustable Photometer Aperture
The photometer is equipped as standard with an eyepiece which is positioned after the bilaterally adjustable vertical and horizontal slits for selecting the field of detection. This allows the user to directly view and adjust the same field of view that the detector sees.
One can replace the eyepiece with a camera (optional) for digital visualization.
Wavelength Selection
The dual wavelengths are selected by means of a diachroic cube (required but not included, select from list) If you simply want to detect with one PMT, the wavelength selection is by means of one inch round filter (optional).
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