Photodetectors that measure the brightness of light sources find use all around us. They are contained within such everyday items as cameras, bar-code readers and laser printers. They are a mainstay in most scientific instruments used to measure light intensity or color.
But there is another class of optical detectors, known as position sensors, which are unrelated to light source brightness measurements, per se. And though these devices find use in a wide range of applications, many technologists are unfamiliar with them.
For example, medical researchers use such position sensors to track the high-speed patterns of human eye movement, and to perform 3-D modeling of human motion. Optical scientists rely on position sensors to align lasers, light sources, and mirrors to within fractions of microns. Position sensor technology is incorporated into ultra-fast, accurate auto focusing schemes for a variety of optical systems, such as microscopes. And such industrial measurements as machine tool alignment and vibration analysis are performed with these detectors.
Rather than quantifying the brightness of a light source, these types of measurements are concerned with finding its position in space. As such, this category is generally referred to as optical position sensing, and includes measurements of: angle, straightness, location, height, centering, surface uniformity, distance, movement and vibration.
UDT Instruments, a Gamma Scientific company, has integrated this detector technology into a number of specialized optical instruments, which, as we shall see, are being creatively applied to a wide range of measurements in science and industry.
Basic Detector Theory
It’s important to understand that optical position sensing instruments aren’t imaging systems like cameras and arrays, since the latter require scanning and processing electronics. To calculate the location of an optical event on their surface with high resolution, a computer and image analysis software are needed. As such, using imaging systems for spatial measurements is relatively slow.
Instead, position sensors “know” the position of a light spot on their surface. The photo current they produce is directly proportional to position. So optical position sensing instruments are used where speed, resolution and simplicity are needed – cameras and arrays, where image detection and analysis are required.
To understand optical position sensing instruments, it’s important to understand the sensors they make use of. These form the heart of the systems, and fall into two basic categories: segmented and continuous.
Segmented Position Sensors
Also known as quadrant and bi-cell detectors, these devices have two or four distinct photosensitive elements separated by a minuscule gap. A light spot illuminating just one element only produces photocurrent in that element.When the spot is translated across the surface of the detector, the energy becomes distributed between adjacent elements. The ratio between the photocurrent outputs from these elements determines the relative position of the spot on the surface.
It’s important to note that the detector only provides position information over a linear distance of the spot diameter. Elsewhere, it is known to be in a specific segment, but not exactly where. Because of this, when working with lasers, defocusing may be required in order to obtain maximum range.
With a segmented device, another spatial consideration is key: the response to movement of a circular spot is non-linear. This is because the ratio of the spot’s movement to the percentage of its area that shifts between adjacent segments is nonlinear.
For these reasons, segmented detectors are best used as nulling and centering devices.And for such applications, their performance is unparalleled. In fact, a repeatability of 0.1μm has been routinely demonstrated.This high resolution stems from the almost perfect response uniformity between elements. Also, with light-level sensitivities approaching one picowatt, segmented devices will work with far dimmer sources than will continuous position sensing detectors.
Continuous Position Sensors
When position sensing applications require measurement over a wide spatial range, continuous detectors are the right choice. The primary difference between segmented detectors and continuous ones is that the latter are single photodiode. There is no gap or dead region between cells.
Continuous position sensing detectors derive position by dividing photo-generated electrons within their substrate, not by profiling intensity distribution on the surface as segmented detectors do. Therefore, a 2-axis continuous sensor acts as a pair of light-controlled variable resistors that measure the X and Y position of an incident light spot.
Compared to segmented detectors, the primary advantage of continuous position sensors is their wide dynamic range: they measure the position of a light spot right out to their edge. It’s also important to note that these sensors determine the centroid of a light spot. This gives them the advantage of being indifferent to a spot’s shape or intensity distribution.
For nulling or centering applications, the spatial resolution of a continuous device is inferior to that of a comparable segmented device. This stems from the lower signal-to-noise ratio of continuous devices. So continuous position sensors work best for measuring a light spot’s movement over a wide range.
Continuous position sensors are available in one and two dimensional configurations, and come in four types—duo-lateral, tetra-lateral, pin-cushion and transparent duo-lateral.
The duo-lateral type has electrodes on both its front and rear surfaces. From the equivalent circuit it can be seen that each position signal is divided into just two parts…but by two separate resistive layers. This approach produces minimal position sensing error and very high resolution.
Tetra-lateral types have four electrodes on the front surface of the photodiode. As such, the total induced photocurrent is divided into four parts by the same resistive layer. Compared with the duo-lateral type, tetra-lateral devices are more non-linear for positions further from their mechanical centers. However, the tetra-lateral devices produce less dark current and have a faster response time. And they are somewhat easier to operate since minimal, or even zero, bias voltage is required.
Pin-cushion devices are basically an improved tetra-lateral, with reduced signal non-linearity at the edges.This is achieved by increasing the photodiode’s surface sensitivity and modifying its electrodes. The pin-cushion device offers all the advantages of the tetra-lateral type, namely, low dark current, fast response, and minimal bias-voltage requirements.
Transparent duo-lateral detectors are essentially the same in principle as duo-lateral. However, they are constructed by depositing amorphous silicon on a transparent substrate. Thus, an incident beam can pass right through the detector after experiencing a small amount of attenuation and diffusion.
Optical Position Sensors and Position Sensing Instruments
Our extensive selection of optical position sensors, position amplifiers/indicators and accessories allows our applications engineers to assemble and configure a tremendous variety of position-sensing systems.