R. James Rockwell, Jr.

ROCKWELL LASER INDUSTRIES

DEFINING THE PROBLEM

The ability of laser protective barriers to resist an incident beam has been frequently reviewed at numerous standards committee meetings. Specifically, the concern of how to rate such devices has been raised.

These discussions have typically concluded with the recommendation that a laser protective barrier testing protocol is needed and that it might focus on the ability of the barrier to withstand beam penetration when exposed for a finite (pre-selected) time period at a maximum incident irradiance level. Exposures ranging from 10-1000 seconds have been suggested.

Such a irradiance - time rating would, then, provide a means, to determine a Penetration Threshold Level (PTL) of a given barrier design and, therefore, provide a measure of whether such a barrier would withstand a specific "real world" exposure scenario.

Previous studies have shown that laser exposures of some barrier designs often display a spot-size dependence in the PTL. This can also be an important factor, and raises the question as to what beam diameter size is the most important in effecting the PTL rating - or - whether a range of diameters is more appropriate.

Currently there is no established testing protocol to establish the laser barrier PTL's. This presentation will present PTL data for various laser barriers for one proposed testing method. Several laser protective barriers were tested using the proposed protocol so as to determine:

• The PTL's as a function of a pre-selected time factor versus the incident laser irradiance for non-focused beam spot size diameters, and
  
  
• The PTL's using different laser types: CW - pulsed - repetitively pulsed, and
  
  
• The PTL's at different laser wavelengths

In this protocol an overall exposure time of 100 seconds was selected. It is argued that during an accidental over-exposure condition, the laser would be turned-off or the beam otherwise terminated within the 100 second time factor. Thus, the testing was done so-as-to determine the highest beam irradiance for which no penetration occurred for an exposure of 100 seconds for a range of beam spot size diameters.

The presentation will present PTL data for various laser protective barrier designs to the beams of Carbon Dioxide, Nd:YAG and Argon lasers.

It was first established in discussions on the Control Measures Committee of the ANSI Z136.2 Standard that the testing protocol might focus on the ability of the barrier to withstand the beam (non-penetration) for a pre-selected time period. This would allow a comparison amongst the various barrier designs for a measure of equivalent performance for a given barrier.  Thus, each laser protective barrier would be tested to establish a Penetration Threshold Level (PTL).

CRITERIA

There is no current national standard for the measurement methods for such penetration threshold levels. There has been recent recommendation that the ANSI Z-136 committee consider establishing a separate standard on Laser Protective Equipment.  One mission of such a new standard would be to recommend a measurement protocol for laser protective equipment (e.g. eyewear, window filters and protective barriers).

LASER BARRIERS AS A CONTROL MEASURE

As the scope of laser controls more frequently include an area barrier as a means of either temporary or permanent protection, it becomes important to determine if the barrier can provide the protection that is needed. Simply stated...will the barrier withstand the laser beam long enough - without penetration - so that the operator can turn off the laser or take other actions to avoid the beam?

The ANSI Z136.1 standard [1] recommends (see section 4.6.4):

HOW TO RATE LASER BARRIERS

One testing protocol would be to establish that no measurable beam penetration occurs:

1. For a maximum incident irradiance level for a pre-selected exposure time (10-100 seconds have been suggested) for a range of beam sizes
   
   
2. The irradiance level where No Measurable beam Penetration (NMP) occurs would be determined. From this data, the highest exposure where penetration does not occur would be established. The PTL could then provide a rating of whether such a barrier would withstand a specific "real world" laser exposure.

Previous studies have shown that laser exposures of some multi-layer barrier designs yield a PTL that is dependent on the laser beam size. This raises the question as to what beam diameter size is the most appropriate in effecting the PTL rating - or - whether a range of diameters may be chosen?

At this time there is no testing protocol that has been established to rate the laser barriers. The protocol proposed here suggests to establish the PTL's for:

1. A time period of 100 seconds for a range of irradiance values and beam spot sizes,
   
   
2. To determine whether different PTL's result for CW, pulsed and repetitively pulsed lasers, and
   
   
3. To evaluate the PTL's at different laser wavelengths

PROTOCOL OVERVIEW

An experimental protocol is recommended that establishes one method to rate the resistance of a laser protective barrier to an incident laser beam.

It should be mentioned that data obtained in previous studies has shown that such laser barriers often display a beam spot-size dependence on the penetration ability. This is an important factor, and raises the question as to what beam diameter is most appropriate - or - whether a range of data for various diameters is needed in the evaluation.

The protocol attempts to establish the irradiance where a protracted protection (i.e.: "resistance") occurs. This point is suggested in the Appendix of ANSI Z-136.1 (Section C2.4) to be a time period of 60 seconds. Others have suggested 100 seconds. For this analysis, 100 seconds was chosen as the time basis for the protocol.

Thus, the lowest irradiance value for a given beam diameter will be determined for which a given barrier provides non-penetration for a time factor of 100 seconds.  Then a parametric evaluation of non-penetration as a function of beam diameter will be done within the power limitations of the available laser equipment.

TEST PROCEDURE

The following exposure sequence is suggested:

1. The required laser wavelength will be selected at the beginning of each series of protective barrier tests.
   
   
2. The protective barriers to be tested will be placed in a stable mount at the position at which the incident beam irradiance was determined.
   
   
3. If deemed necessary, a band-pass filter will be inserted in the beam path so-as-to assure that only radiation at the laser wavelength will be measured.
   
   
4. Laser power will be first adjusted so that initial tests are performed at levels where penetration definitely occurs.
   
   
5. The incident beam diameter will be established at this laser power setting by the aperture transmission method. The beam diameter and area will be determined at the position at which the protective barrier is to be tested, assuring that the measured diameter is the incident diameter at the protective barriers surface.
   
   
6. The beam irradiance will be calculated for those conditions of beam power and diameter size.
   
   
7. The incident laser power (irradiance) will then be decreased in appropriate increments until beam penetration through the barrier does not occur 100 seconds or less.
   
   
8. Care will be taken to maintain a constant incident beam diameter on the protective barriers surface.
   
   
9. When new test points are selected, care will be taken to avoid repositioning the protective barriers in the exact same location as a previous test (even if no visible effect is noted on the barrier).
   
   
10. Since some barriers have designs which may contain materials which can retain a thermal buildup for a long time, measurement of the inner barrier temperature may be appropriate so that equivalent temperature conditions prevail from test to test.
    
    
11. The Penetration Threshold Level (PTL) is established as the highest irradiance for which no-penetration occurs for an exposure of 100 seconds for a given beam spot diameter.
    
    
12. Where possible, the test and evaluation procedure will be repeated on a minimum of three protective barriers of the same type.

INSPECTION AND EVALUATION

Effects noted would be:

1. First Visible Damage (FVD): Any visually observable change or structural alteration in the protective barriers surface (vis: melting, pitting, cracking, discoloration...etc.) that occurs during or following the exposure.
   
   
2. Flame, smoke - and sign of thermal distortion or fumes.
   
   
3. Penetration Threshold Level (PTL): the beam breakthrough of the material.

TEST METHODS

Beam Penetration Studies:

The beam diameter was determined using different size apertures to reduce the power to 63.2% (1/e) of the total power. This diameter was chosen because it would be consistent with the hazard measurement requirements of the ANSI and FDA/CDRH standards. The input power measurement was made at the barrier position using appropriate laser output measurement equipment.

The power was monitored using a Coherent Labmaster with appropriate measurement head.  Exposures were timed using an auto-timed shutter. Any barrier penetration noted during the exposure would signal the laser operator to cease laser operation. This time was recorded as the time of penetration. Following any given measurement, the laser power would be re-measured to reconfirm calibration.

The highest irradiance for which no-penetration occurred for an exposure of 100 seconds at a specified exposure diameter was established as the Penetration Threshold Level (PTL) for that laser type and beam spot size.

SAMPLE LASER PARAMETERS:

The laser could be arranged at irradiance levels over a range, for example, from 50 to 300 W/cm2.  Burn through (penetration) should be noted at the time it occurs at each power level tested. Experience has shown that one can achieve data that will have different "burn through" irradiance levels for different beam spot diameters. This allows one to generate a graph of spot size - vs - laser beam irradiance (W/cm2) for a fixed maximum penetration time (100 sec.). (See attached data)

Usually the multi-layered laser-resistant barriers will not be penetrated up to a specified irradiance for very long times and, then, once a critical irradiance level is reached, then burn through will occur almost immediately.

DATA SUMMARY

This report covers only examine exposures for two CW lasers for a range of irradiance values and beam spot sizes. The two lasers and wavelengths are:

1. Nd:YAG @ 1064 nm
   
   
2. Argon @ 514 nm

The other factors must await yet further testing.

Thus, the testing reported here was done so-as-to determine the highest laser beam irradiance for which no measurable penetration occurred for a total exposure of 100 seconds for beam spot size diameter ranging from 4 - 6 mm.  It is argued that during an accidental over-exposure condition, the laser would be turned-off or the beam otherwise terminated within the 100 second time factor

PTL DATA

Four multi-layered laser barriers designs were tested. The tests showed that the barriers were not penetrated until a critical irradiance level is reached for a given beam size. Then, penetration occurs almost immediately!

The data is given in the accompanying data plot in Figure 1.

Otherwise, the barrier would withstand the exposure for at least 100 seconds - if not significantly longer.

FIGURE 1            

CONCLUSION

The laser barrier testing protocol used appears to provide the desired end point.  A rating method for laser barriers that indicates the maximum exposure level where no measurable beam penetration occurs, and the PTL value seems to provide a very simple rating which directly relates to typical "real world" laser exposure conditions.

REFERENCES

1. American National Standards Institute, American National Standard for the Safe Use of Lasers: ANSI Z-136.1  (1993),  Publisher: Laser Institute of America, Orlando, FL, 1993.
   
   
2. James Rockwell, Jr., James F. Smith and Wm. J. Ertle,  Playing it Safe with Industrial Lasers, Photonics Spectra, Vol 29, No. 4, April, 1995.