High Accuracy Laser Power Measurement for Laser Pointers Safety Assessment

Abstract

Laser pointers are common tools for teachers, students, and researchers as pointer devices in classrooms or meeting rooms. Some people use laser pointers for toys, hobbies and entertainments. For safety concern, these laser pointers are designed to emit laser power level below 5 mW as restricted by many countries regulations. However, advances in laser technology, allowed the production of low low-cost laser pointer devices, at a wide range of visible wavelengths, delivering considerable light output power, which are available for the general public. In fact it is now common, to have laser pointers and laser gadgets of all colors and power range, from few mW to several watts available to buy. As a result, laser pointers are in the hands of uninformed people about the potential injuries that can arise from the handling of these devices. This is resulting in a worldwide increase of retinal injuries reporting. In this research, the laser power levels of randomly purchased laser pointers in Thailand markets will be assessed in order to check the accuracy of their laser safety labels, using a dedicated laser pointer power testing kit developed at National Institute of Metrology (Thailand). A set of twenty laser pointers, red, green and violet, randomly acquired in market have been assessed with respect to their power output level and emitted wavelengths lines. The limits imposed by The US Code of Federal regulations for lasers devices will be used as a reference.

Appendix

4.1.1 Measurement Results

Average laser output power outcomes from red, green, and violet laser pointer devices are shown in Tables 4.5, 4.6 and 4.7.

Table 4.5 Average laser output power from red laser pointers

Measurement Uncertainty. Measurement uncertainties were estimated following the guideline from Taylor and Kuyatt [16] and Evaluation of measurement data, Guide to the expression of uncertainty in measurement (GUM) [17]. There are 2 types of uncertainties associated with measurement, type A standard uncertainty, arising from statistical analysis of a series of observations, and type B standard uncertainty, coming from other than statistical analysis, for this work, in this case related with the measuring system itself.

Type A Uncertainty. Calculated from standard deviation of 5 laser power measurements, $S_r$. Type A standard uncertainty corresponds to standard deviation of the mean $\frac{S_r}{\sqrt{5}}$.

Table 4.6 Average laser output power from green laser pointers

Table 4.7 Average laser output power from violet laser pointers

Fig. 4.12

The laser power calibration experimental setup

Type B Uncertainty. Obtained from uncertainty of measurement system. In this work, the estimated system uncertainty, $u_s$, is

$$\begin{aligned} u_s = \sqrt{ u_d^2 + u_f^2 } \end{aligned}$$

(4.5)

where $u_d$ is the power detector uncertainty of and $u_f$ is the bandpass filter uncertainty. The combined standard uncertainty of measurement, U, is

$$\begin{aligned} U = \sqrt{ u_s^2 + \frac{s_r^2}{5} } \end{aligned}$$

(4.6)

The uncertainty of the power detector is obtained by calibration of laser reference standard, by direct comparison with the laser Calorimeter (traceable to SI unit). Calibration was carried out at 633, 515 and 488 nm wavelengths. The calibration experimental setup is shown in Fig. 4.12. The calibration factor ($C_n$) is

$$\begin{aligned} C_n = \frac{P_u}{P_s} \end{aligned}$$

(4.7)

where $P_u$ is power measured by the unit under test (thermopile detector) [W], and $P_s$ is the laser power measure by the standard [W]. The calibration results are shown in Table 4.8.

Table 4.8 Power detector calibration results

The uncertainties of the bandpass filters were estimated from transmission calibration measurements using a Nd:YAG laser light source at 1064 nm and 532 nm. The calibration experimental setup is shown in Fig. 4.13. For the 1064 nm bandpass filter, the transmission value considered was the ratio of laser power reading on a standard pyroelectric detector with and without the filter. The calibration attained values are displayed shown in Table 4.3. Concerning the the 800 nm filter, the light transmissions at 1064 nm and 532 nm were checked. The calibration results are in Table 4.9. By using (4.6), a 1% of total uncertainty can be obtained.

Fig. 4.13

The bandpass filter calibration experimental setup

Table 4.9 Bandpass filter calibration result