Optical Measurement & Laser Safety

Background

Lasers have been used in the entertainment industry since the mid-1960s and every year bigger and better shows are put on. Lasers are classified by wavelength and maximum output power into four main classes based on their potential for damage to people. Class 1 lasers present no hazard during normal use through to Class 3 lasers which present a sever hazard for eyes and skin.

Laser light shows can often use Class 3B lasers or higher which are harmful to the human eye if exposed directly. It is possible to use these lasers in public displays as a number of factors which reduce risk are taken into consideration including the number of reflections off objects and the amount of time the laser is shining at any one object (most lasers are scanning continuously thus reducing the power focused at any one point).

We were asked by the HPA to help develop test equipment which could be used to help test lasers systems to ensure that they conform with safety legislation. In addition the test system would be used to examine how the interaction of multiple laser beams can affect crowd safety, something which until recently has been very hard to do. In most laser light shows multiple lasers are used in the entertainment, the effect of the beams intersecting can form interference, potentially doubling the lasers power at the target.

Requirements

In order to realistically monitor laser safety for humans a laser detector system based on the human head and eyes was developed. The detector was made up from two photo detectors 60mm apart (the average distance between eyes) with a 12mm aperture (the average diameter of a fully dilated human pupil). This detector could then be mounted on a tripod at head height and placed in amongst a crowd to see the typical number of laser strikes and measure the power and duration of each strike.

Data was sampled at 250ks/s on each channel (left and right eyes), giving a minimum detectable laser pulse width of 4μs. Some lasers are pulsed (a technique which is used to reduce the total output power) however despite lowering the transmitted power it doesn't mean that no damage is being done. The software had to capture individual pulses (considered longer than 4μs) as well as pulse trains. In order to do this an internal buffer is used to store all the data from each channel, it is then analysed point by point and the level threshold checked, when it is exceeded the pulses are logged. Pulses of up to 10s (per channel) can be logged using the system which is designed to run on a standard laptop.

Lasers light shows are popular but can be dangerous

Lasers light shows are popular but can be dangerous

Whilst data is being logged the software uses various algorithms to calculate the Maximum Potential Exposure (MPE) levels as well as the Hazard Ratio in pseudo real-time.

Results

The system developed has been used by the HPA in various studies. A number of additional calculations were added to enable the ratios to be calculated based on combinations of laser pulses (opposed to considering all pulses within a timeframe). The system was also expanded so that data could be collected locally with key safety information being transmitted via Bluetooth to a user located elsewhere in the arena.

Used in this project

Software

 LabVIEW, DAQmx

Hardware & Interfaces

NI USB DAQ, Bluetooth

Skills

High speed data processing, wireless data communication and logging, remote monitoring and control, LabVIEW programming