By Dick McCreary, Ohio Calibration Laboratories
The Ophir Nova or Laserstar series power meters, along with the appropriate photodiode sensors, comprise optical instrumentation we use on a daily basis for technical support and calibration of police lidar devices.
With ever increasing frequency, police agencies throughout the world are enjoying the pinpoint accuracy and reliability of laser-equipped lidar instruments for vehicle speed enforcement. Crime scene and vehicle crash reconstruction can be more easily accomplished using these devices as accurate measuring tapes and outputting their measurements to a portable data collector or palmsized computer
Other features offered by these instruments include some relatively straightforward software. This means that, for the first time, police can actually measure the speed, direction of travel, and separation distance between two moving vehicles.
How many times have you been driving normally at a reasonable speed only to encounter someone who was obviously either more important or, in any event, was in such a hurry as to be unwilling to maintain a reasonable space between you and their vehicle? How often have you felt the urge to speed up and perhaps the tailgater will begin to back off? But, you accelerate to a speed that you aren't comfortable driving (you obviously don't want to get a speeding ticket) but the moron is still stuck to your rear bumper. About all you can do is pull over if the road permits and let the nut job pass you. Another urge may be to slam on the brakes and hope the tailgater is able to stop in time. But don’t try this at home; all that will do is promote road rage, something you should try to avoid at all costs.
Measuring the Distance Between Vehicles Now, for the first time, a prominent lidar manufacturer has come out with an instrument that allows police to accurately and reliably measure the actual separation distance between two vehicles and, based on their speeds, automatically calculate their closing time accurate to a tenth of a second.
Based upon this capability, police are beginning to employ these specialty instruments in large numbers as following too closely is one of the major contributors to traffic crashes, injuries, and deaths.
These devices are required to be periodically maintained and documentation to that effect is often required to be presented in court. Independent lidar certification companies depend on Ophir-Spiricon products to accomplish that task.
These instruments must make accurate measurements of power output, wavelength frequency, and beam dispersion angle. Unlike conventional lasers, in order for the instrument to be able to be reliably aimed at a distant target, in this case a motor vehicle, the beam dispersion angle has be to intentionally increased to dramatic values as compared to traditional laser optical standards. Specifically, the beam is intentionally dispersed to a diameter of three feet at one thousand feet from the instrument.
Being in the near infrared spectrum, it is invisible to the motoring public and it's extremely low power allows the instrument to be deemed Class 1 eyesafe by the FDA.
The beam is pulsed at a low rate of between 100 and 350 Hz with the time of flight of the laser beam being timed by an internal electronic counter. In the case of the Distance Between Cars (DBC) model, the device’s internal program, after determining the distance, direction of travel, and velocity of the first car, interpolates the data to the first vehicle's calculated location immediately after determining the same data on the second vehicle. In so doing, the instrument can, in effect, look ahead in time and predict the location of the first vehicle vs the second.
Withstanding Court Challenges
As one can appreciate, all this relatively sophisticated technology, especially to the general public, can be subject to court challenge. Therefore, at times, the technology must be backed up by expert witnesses who can analogize to the satisfaction of the court, the basic principles of operation and the inherent accuracy of the concept.
But it all starts with accurate and dependable test instrumentation. Using the Ophir Nova or Laserstar power meters, in concert with Ophir's excellent data collection software, StarCom, we are able to make permanent data file records of each instrument's certification profile and include hard copies of the charts generated by each instrument under test for presentation in the courts at some future date.
We have determined that each instrument has it's own unique signature which is repeatable over time, somewhat like a fingerprint. Having this baseline of expectations, we can far more easily diagnose and pinpoint any potential problems or malfunction that might arise in the future by comparing the current digital fingerprint to one taken previously.
All things considered, the ease of use of the Nova or Laserstar in combination with StarCom results in a low cost but impressive data collection product that is convincing to the nontechnical and hard to challenge by the technically gifted.
Next Steps: Adding Beam Profiling
We are planning to incorporate a beam profiling addition to our facilities as soon as the economics of today's financial problem permits. The issue facing our industry, however, is not if the beam profiling camera will work. The enormously wide dispersal angle of the laser beam calls for special consideration to permit the camera to accommodate the wide beam width.
We are planning to work with Ophir-Spiricon’s optical engineers to develop an optical convergence assembly that will reduce the beam by a known factor and thus be able to easily view the beam's profile and calculate it's divergence angle as is required by a growing number of agencies across the nation.
Dick McCreary can be reached at Ohio Calibration Laboratories, 6663 Huntley Rd Suite E, Columbus OH 43229-1038, phone (614) 840-8500, fax (614) 840-9360.