Laser Safety

Laser light is monochromatic, directional and coherent. The combination of these three properties makes laser light focus about 100 times better than ordinary light. The retinal hazard region is considered to span from 400 nm to 1400 nm, thus covering both the visible and the infrared regions.

Some common causes of laser injuries:

· Inadequate training

· Alignment performed without adequate procedures

· Failure to block beams or stray reflections

· Failure to wear eye protection in hazardous situations

· Failure to follow approved standard operating procedures or safe work practices

The beam divergence is calculated as shown below:

[rad]

Laser Safety of our RBS subsystem

A typical device

A diode laser is used for scanning. The laser is set to operate at 5 mW optical power (I_F=50 mA at 25ºC). Laser power is additionally decreased by means of an attenuating filter 5:1. The divergence of the laser beam coming out of the laser is 9.5⁰ (parallel) and 23⁰ (perpendicular), i.e. an average divergence of 16.25⁰. The optical design uses only the inner 4.3⁰ diverging beam. Assuming a normal (Gaussian) radial distribution, and using integration of the volume enclosed by a rotating Gaussian, it can be shown that the only one third of the laser power is thus captured, i.e. 5 mW/(3x5) = 0.333 mW. This power is focused by the optics into a point focus which is 600.3 mm away from the pupil, and is directed from there in a beam of divergence 1.0784º (15.0 mrad) to an exit pupil of diameter 11.3 mm (100 mm² = 1 cm² ), where the pupil will be during scanning. This results in 0.333 mW/cm² at the subjects' pupil.

The MPE

The Maximum Permissible Exposure (MPE) is determined using the American National Standards Institute (ANSI) Z-136.1 safety standards [1]. We used Table c (MPE for a point source ocular exposure to a laser beam from 700 nm to 1400 nm). For λ=700-1050 nm and exposure duration of 10 to 30,000 s, one obtains for the retina MPE irradiance

MPE = C_A x 10^-3 W/cm² (1)

From Table 6a, the factor C_A for the same wavelength range (λ=700-1050 nm) is calculated as C_A = 10^{(0.002(λ-700nm))}. Using λ=785 for our laser, we obtain C_A =1.48, and hence

MPE = 1.48 x 10^-3 W/cm² = 1.48 mW/cm² = 1,480 µW/cm². (2)

The irradiance of 0.333 mW/cm² (or 333 µW/cm²) at the subjects' pupil is thus well below the established the American National Standards Institute (ANSI) Z-136 safety standards of 1.48 mW/cm² for a fixed (CW) laser for an indefinite period.

An alternative calculation was done for scanning periods of less than 10 s, following the same ANSI Z-136 standard for direct ocular exposures (intrabeam viewing) from a laser beam, as summarized in [1], Table 5c, and in [2], Table 1, page 261-283, and in particular Table 8-1 on p 262 and Figures 8-3 and 8-8. These calculations utilize the formula for MPE

MPE = 1.8 x C_A (t/(t^1/4)) x 10^-3 J/cm² = 1.8 x C_A (t^0.75)) mJ/cm² (3)

for exposure times of 5 µs to10 s. A 10 s lasting exposure would therefore produce radiant exposure of

MPE:H = 1.8 x 1.48 x 10^0.75 x 10^-3 J/cm² = 1.8 x 1.48 x 5.623 x 10^-3 J/cm² = 15 mJ/cm² (4)

and

MPE:E = MPE:H / t = 1.5 mW/cm² for t = 1 s (5)

A calculation similar to the one in equations (4) and (5) results in higher MPE:E for shorter exposure times: 1.78 mW/cm² for t = 5 s, 2.24 mW/cm² for t = 2 s, and 2.66 mW/cm² for t = 1 s. It should be mentioned that the examination to be done by the present system will most likely be completed within 2 to 10 s.

These calculations further assume a limiting aperture of 7 mm (0.38 cm²). It is important to note that to provide the most conservative possible calculations, we made the assumption that the laser is stationary. This allows for the safety to be assessed even in the case of inadvertent exposure due to the unlikely event that the laser remains illuminated, the scanning motor fails, and an individual positions the eye at the face of the instrument and stares at the immobile laser instrument, either incidentally viewing, or purposefully staring, for 30,000 s without moving or blinking. With these worst-case assumptions, the limit for the 785 nm laser is 1.48 mW/cm², equivalent to 0.562 mW going through a 0.38 mm² pupil. If the laser is being scanned, the subject is at the normal operating position, or the subject looks away or blinks during the 30,000 s time period, the safe threshold will be considerably higher.

The RBS portion of the power at the exit pupil is ca. 0.333 mW/cm²_, equivalent to 0.127 mW going through a 0.38 mm² pupil. These values are nearly 4.4 times lower than the MPE for exposures lasting longer than 10 s or for an indefinite period (eq. 2), and even lower for exposures lasting less than 10 s (eq 4).

References

[1] American National Standard for Safe Use of Lasers, ANSI Z136.1 – 2014. Revision of ANSI Z136.1-2007. Laser Institute of America, 2014.

[2] D. Sliney, M. Wolbarsht, “Safety with Lasers and Other Optical Sources,” Plenum Press, New York (1980).

Laser Safety Levels (ANSI Z-136)