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Flashing-Light Photometric Standards

Airplane with inset of two flashing lights

Flashing lights are widely used in many signaling applications in aviation, marine, and land transportation. To meet the industry needs, NIST established a facility for flashing light calibration under support by FAA in 1997 and maintains the unit of lux second and provides a calibration service for flash photometers including aircraft anti-collision light meters. An important issue raised recently is the existence of a few different formulae for effective intensity measurement, and an international standardization is becoming urgent. NIST recently studied the differences in the three different formulae - Allard, Blondel-Rey, and Form Factor method. The problems with these methods have been clarified by simulation on various forms of pulses. As a result, NIST developed the Modified Allard method, which NIST is proposing this method for international standardization.

NIST Facility for flashing light calibration

Flashing λ lights are widely used in many signaling applications in aviation, marine, and land transportation. The photometric quantities of flashing light includes luminous exposure (lux second), time-integrated luminous intensity (candela second), and luminous energy (lumen second). Corresponding radiometric quantities are radiant exposure (W m-1 s), time-integrated radiant intensity (W sr-1 s), and radiant energy (Joule). NIST established a facility for flashing light calibration under support by FAA in 1997 and maintains the unit of lux second and provides a calibration service for flash photometers including aircraft anti-collision light meters [1]. The units are maintained on a set of four flashing-light photometers.

NIST flashing-light standard photometer
Electronics units

Electronics units

 Photometer heads

Photometer heads

Two different approaches were taken to calibrate flashing-light standard photometers: 1) based on electrical calibration of the current integrator (Electrical method), 2) based on electronic pulsing of a steady-state photometric standard (Pulsed photometry method). The principles of these two methods are illustrated in figure below.

Two methods employed in the NIST realization of luminous exposure unit.

Schematic of electrical method
Output Charge: $Q [{\rm C}] = C \cdot V$
Luminous exposure: $H [{\rm lx \cdot s}] = Q/s$
Photometer responsivity: $s_{\rm H} [{\rm V/(lx \cdot s)}] = s/C$
 
Schematic of pulsed photometry method Luminous exposure: $H [{\rm lx \cdot s}] = E \cdot T$
Photometer responsivity:

Effective Intensity

 In addition to these physical quantities, another quantity, effective intensity, is commonly used in transportation applications. Flashing lights, such as aircraft anti-collision lights, marine aids-to-navigation lights, obstruction lights, and emergency vehicle warning lights, are specified for effective intensity (cd). Effective intensity is defined as luminous intensity (cd) of a steady light, of the same relative spectral distribution as the flashing light, which would have the same luminous range (or visual range in aviation terminology) as the flashing light under identical conditions of observation [2]. Pulses having shorter duration and high peak are more conspicuous than slower pulses with low peaks having the same physical intensity (candela second). The effective intensity is to address such temporal response of human visual system. There have been three well recognized formulae available to calculate effective intensity: by Allard in 1876 [3], Blondel-Rey in 1911 [4], and Form Factor method [5].

These different formulae give different results depending on pulse shape and duration, but they give the same effective intensity value for very short pulses (~1 ms or less). Xenon pulses are currently widely used for warning lights, and as long as each pulse is a single peak pulse, this has not been a problem. The current NIST calibration service is serving for such needs for measuring flashing light products to measure xenon flash pulses. However, for warning lights that emit a train of flashes (e.g., one visible flash consisting of 2 or more flashes in short intervals), there have been serious discrepancies between results from the different formulae. Also, recently, an increasing number of flashing warning devices using LEDs are produced. Since LED flash devices have much longer duration (~100 ms), the value of effective intensity changes depending on which formula is used. Different formulae are used in different countries and in different applications, and the effective intensity values cannot be compared. This is becoming a serious problem, and an urgent need for international standardization on the definition of effective intensity has recently been addressed. Some efforts toward such a standardization have started in Commission Internationale de l'Éclairage (CIE) [6] and in American Society for Testing and Materials (ASTM) [7].

NIST recently studied the differences in the three formulae calculating effective intensity of various waveforms of flash by simulation analysis [8]. The study demonstrated that there are problems with all the three methods, particularly for multiple flashes and some specific forms of pulse. The results of the study demonstrated that the Form Factor method fails for a train of multiple pulses and some pulse with a very narrow peak. The results also indicated that Allard method would work best with any forms of pulses but it had one problem that the results for rectangular pulses did not agree with that of Blondel-Rey formula, which is believed to be accurate for rectangular pulses. We solved this problem by modifying the Allard method, which is called the Modified Allard method [8]. On the simulation, the new formula works most reasonably for any forms of pulses among the three available methods. Modified Allard method works in a similar manner as an electronic low pass filter. It does not require the waveform of pulse and can be measured directly with simple analog electronic circuit [9]. Visual experiments are expected to verify the correlation of results with visual perception.
 

Blondel-Rey (1911)

 Blondel-Rey (1911)
(a) and (b)
$I_{\rm e} = \frac{\int_{t_1}^{t_2} I(t) {\rm d}t} {a+(t_2-t_1)}; \quad a= 0.2~s$

t1, t2 are determined to satisfy

$I_{\rm e} = I(t_1) = I(t_2)$

(c) This is solved as

$\int_{t_1}^{t_2} [I(t) - I_{\rm e}] {\rm d}t = a \cdot I_{\rm e}$

Form Factor Method
Schmidt-Clausen (1967)

 Form Factor Method, Schmidt-Clausen (1967)
$I_{\rm e} = \frac{I_{\max}}{1+[a/(F\cdot T)]}$

$F = \frac{\int_0^T}{I_{\max} \cdot T} \quad (a= 0.2~s)$
This is transformed to:
I_{\rm e} = \frac{\int_0^T I(t) {\rm d}t}{a+\Delta T};   $\Delta T = \frac{\int_0^T I(t) {\rm d}t}{I_{\max}}$
 

Allard (1876)

 Allard (1876)
Instantaneous effective intensity
i(t) is solved by the equation:

$\frac{{\rm d}i}{{\rm d}t}=\frac{I(t)-i(t)}{a}; \quad a=0.2~s$

Ie is the maximum of i(t).
This is solved as

$i(t) = I(t) ^*q(t); \quad q(t) = \frac{1}{a} ~ {\rm e}^{- t/a}$

q(t): visual impulse response function.
*: convolution

Modified Allard (Ohno and Couzin)

 Modified Allard (Ohno and Couzin)

$i(t) = I(t) ^*q(t)$

with modified q(t):

$q(t)= a/(a+t)^2$

References

  1. Ohno, Y. and Zong, Y., Establishment of the NIST Flashing-Light Photometric Unit (91 kB) PDF, Proc. Photometric Engineering of Sources and Systems, SPIE, 3140, 2-11 (1997).

  2. International Lighting Vocabulary, CIE Publication No.17.4, IEC Publication 50(845) (1987).

  3. Allard E., Mémoire sur l'intensité et la portée des phares, 62-73, Imprimerie Nationale, Paris (1876).

  4. Blondel, A. and Rey, J., Sur la perception des lumiéres bréves à la limite de leur portée, J. de Physique, juillet et aout, 643 (1911).

  5. Schmidt-Clausen, H.J., Concerning the perception of various light flashes with varying surrounding luminances, Darmstädter Dissertation D17, Darmstadt Univ. of Technology, (1968).

  6. CIE Technical Committee 2-49, Photometry of Flashing Lights, Chair: Y. Ohno.

  7. ASTM E12 Color and Appearance, Subcommittee 11 Visual Methods, Task Group 05 Flashing Lights.

  8. Y. Ohno and D. Couzin, "Modified Allard Method for Effective Intensity of Flashing Lights (913 kB) PDF," Proc. CIE Symposium'02, Veszprem, Hungary, CIE x025:2003, 23-28 (2003).

  9. Y. Ohno, "Physical Measurement of Flashing Lights - Now and Then (916 kB) PDF," Proc., CIE Symposium’02, Veszprem, Hungary, CIE x025:2003, 31-36 (2003).

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For technical information or questions, contact:
Yoshi Ohno
Phone: (301)-975-2321
Email: ohno@nist.gov 		    Cameron Miller
Phone: (301)-975-4713
Email: ccmiller@nist.gov

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Online: July 1999   -   Last updated: May 2007