Measure UV to IR optical radiation with the Merlin™ Digital Lock-in Radiometry System and its family of calibrated detectors. The unique Smart Ranging™ feature gives you seamless gain switching. Over seven decades of input signal values, sub-microvolts to volts, are easily accommodated without electrical transients. We offer single and dual channel RS-232 and GPIB-488 Merlin™ instruments; each channel includes a reference capability. A broad selection of calibrated detectors, which have preamplifiers matched to Merlin™, are available. Merlin™ works with chopped radiation, repetition rates from 8 to 1100 Hz. An external chopper wheel is required; Merlin™ provides power and control for the chopper.
Why Merlin™?
What makes up a Merlin Digital Lock-in System?
Typically three components make up a Merlin System:
- Merlin Control Unit
- Detector(s) with preamplifier
- Optical Chopper
These three components were designed to work together as a system. When you plug the detector into the Merlin Control Unit, Merlin identifies the detector and, if you are using a calibrated detector(s), downloads the calibration information.
Merlin Control Unit
Merlin is a true digital lock-in. A powerful signal processing computer performs the demodulation and filtering. This allows greater flexibility and fidelity than available from analog lock-ins. Select single or dual phase operation and single or double time constant (one of two pole filtering), as needed. The impressive computational capability allows immediate signal ratioing and conversion, and forms the basis for Smart Ranging™.
The Merlin Control Unit is the heart of a Merlin Radiometry System. The Control Unit drives the chopper and acts as a lock-in amplifier to retrieve the signal and reject unmodulated background radiation. It then computes the signal value using previously loaded calibration data. The signal value is displayed in selected units. We offer single channel and dual channel Merlins. The dual channel models support two detectors, but only one operates at a time.
Optical Chopper
The chopper modulates the radiation to be measured. We offer an open and two enclosed versions, with a selection of chopper blades. You don't need a separate chopper controller (or power source); it's built into the Merlin Control Unit. Use the sync input if your signal is modulated by other means. Your choice of chopper wheel (not included with any of the Chopper Heads) depends upon your desired chopping frequency.
The 75154 Open Chopper is the most economical choice if you don’t require an enclosed beam path. The 75151 and 75152 are both enclosed chopper heads. Choose the 75151 if you are going to flange mount the chopper to other Oriel instruments; choose the 75152 if you are rod mounting the chopper head or hard mounting to an optical table.
Choosing Your Chopping Frequency
Often, it is enough to simply chop the optical beam; chopping frequency is not important (provided it is below the 1100 Hz limit of Merlin™ DSP). However, there are certain factors that may suggest particular chop frequencies for optimizing your system. One common practice, removing power line generated noise from the system, requires chopping at an odd multiple of 1/2 the line frequency in your geographic region. For example, in the United States, 90 Hz is routinely used because it is the third multiple of 30 Hz, one half the 60 Hz line. You will need to use higher chopping frequencies to follow faster changing signals.
Another consideration is detector sensitivity to chop frequency. Pyroelectric and PbS detectors, for example, work better at low frequencies, preferably below 200 Hz. On the other hand, PbSe detectors have a better response at higher chopping frequencies. The best way to determine the correct chop rate is to connect your system and optimize according to your particular conditions. In addition to the "1/2 Line Law" use this rule of thumb to get started: use the highest chop frequency and lowest corresponding time constant that will give you an adequate signal to noise ratio; this will speed your data collection rate.
Beam Size and Calibration
Merlin calculates the 1st harmonic content of the chopped signal and this provides the RMS value. Relation of this RMS value to the peak-to peak value of the signal varies depending on the ratio of the beam size to the chopper wheel aperture size. For a very small beam, i.e. a HeNe laser beam passing through a 5 aperture wheel, the signal is almost a square wave, and the resulting ratio of the measured RMS voltage to peak-to-peak voltage is 0.4502. When the enclosed chopper wheel is illuminated by a totally uniform beam, a triangular wave shape results, and the resulting RMS is 0.3113 of the peak-to-peak value. Other illumination conditions produce intermediate values of the RMS factor. When you order a calibrated Merlin detector, you will receive data based on the square wave, small beam, experiment set-up. If your measurement conditions are different, you will need to modify the calibration factor to reflect the change in RMS multiplier.
Detector
We offer the following Merlin™ detectors
Table 1 Detectors for Merlin™
|
Detector Type |
Usable Wavelength Range |
| Photomultipliers |
160 to 1100 nm |
| Silicon |
185 to 1100 nm |
| InGaAs |
800 to 1700 nm |
| Ge |
700 to 1800 nm |
| PbS |
0.7 to 3 µm |
| PbSe |
1 to 5 µm |
| Pyroelectric |
200 nm to 40 µm |
| HgCdZnTe |
3.0 to 6 µm |
Smart Ranging™
Merlin's™ Smart Ranging™ feature simplifies measurements. The gain setting is automatically and seamlessly selected. Merlin™’s wide dynamic range, multistage, very low noise amplifier chain is sequentially sampled at each stage many times during each cycle and the highest, non-saturated voltage is utilized for analog to digital conversion, A/D. This means you are ensured of maximum utilization of the A/D system resolution for every reading. The 40 bit accumulator and registers of the DSP board create an effective theoretical dynamic range of 1012, but noise levels bring it down to 107.
Full Capability Second Channel
Automation is much easier with a dual channel Merlin Control Unit. The second channel allows you to switch to another detector automatically so you can cover a much broader wavelength range. All the same features that are accessible on your first channel are available on the second. A dual channel Merlin and Cornerstone™ 260 1/4 m Monochromator make an excellent combination since the Cornerstone™ has two output ports. The optional TRACQ™ Software will control both instruments, acquire, process and display the data.
Internal Software Does the Work
Merlin™'s electronics provide you with full control of the system's parameters; Merlin™'s internal software allows you to plug in the detector and take basic measurements with minimal set-up. This combination of ease and versatility comes from our user interface. You can control Merlin™ from a menu structure accessed via the instrument's front panel. Four function keys and a keypad give you complete control of all the options. The LCD displays your data in W, W cm-2, dBm or V. To look for a peak signal you can use the display's analog bar scale or scope display with rapid update.
RS-232 or IEEE-488 (GPIB) Communications
Collecting data is simple, with Merlin. The option of either the RS-232 or IEEE-488 (GPIB) communication boards covers most possibilities for fitting Merlin into your computer controlled system. Either of these interface boards allow you to upload system parameters, calibrate the signal, or download the collected data. If you're using an Oriel Cornerstone or MS257 Monochromator with a Merlin, you can command the entire system through TRACQ, our Data Acquisition and Radiometry Software.
