# Presentation: 745111

## Analysis on Effective Intensity of Flashing Lights and 

- Yoshi Ohno, Ph. D.
- National Institute of Standards and Technology, USA
- 100 Bureau Drive, Stop 8442, Gaithersburg, MD 20899-8442
- Email: ohno@nist.gov
- Dennis Couzin
- Avery Dennison
- 6565 Howard, Niles, IL 60714
- Email: dcouzin@stimsonite.com

- TRB 16th Biennial Symposium on Visibility and Simulation
- Iowa City, June 2-4, 2002

**Analysis on Effective Intensity of Flashing Lights and **
**Modification of Allard Method**

## Luminous intensity of a fixed (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. 

**Effective intensity **(of a flashing light)
- Unit: **candela **(cd)
- Symbol for quantity: *I*e

- ILV (CIE 17.4 / IEC 50(845) 11-18

## Formulae for Effective Intensity of Flashing Lights

- 1. Allard (1876)
- 2. Blondel-Rey (1911)
- 3. Blondel-Rey-Douglas (1957)
- 4. Schmidt-Clausen (Form-Factor Method) (1967)

- Results vary significantly between different methods, for various waveforms of pulses including a train of pulses.  (For very short flashes, all methods give the same results.)

## Blondel-Rey (1911)

## Blondel-Rey (1911)

**Blondel-Rey (1911)**

- This is solved as

- t1, t2 are determined to satisfy
*            **I*e*=I(t*1)=*I*(*t*2)

**Blondel-Rey-Douglas (1957)**

- It is solved as

- where* I*e*=I(t*1)=*I*(*t*2),  *a*=0.2 s

- (extension of B-R for a train of pulses)

## Practical Aspects of Blondel-Rey and Blondel-Rey-Douglas

- Require the waveforms of the pulses.  Waveforms must be measured accurately.
- Require iterative solution, thus need a computer.  It cannot be realized by simple analog circuits.

- Thus, difficult to produce portable (hand-held) flash photometers.

## Form Factor method

**Schmidt-Clausen (1967)**** **

- (*a* = 0.2 s)

- This is transformed to:

_**Practical Aspects**_

- Calculation is much simpler than Blondel-Rey(-Douglas).
- Requires only the integral and the peak of the pulse.  (Waveform not needed)
- Can be realized with analog circuits consisting of a current integrator and a peak-hold circuit.

## Allard (1876) ... rarely used

- Instantaneous effective intensity *i*(*t*) is solved by the equation:

*I*e is the maximum of *i*(*t*).

- This is solved as

_**Practical Aspects**_

- Requires only the peak of the convolution. (Waveform not needed)
- The convolution can be achieved with a simple R-C filter circuit.

- Works through DC.  Calibration can be done by using a steady-light luminous intensity standard.

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

## Standards concerning Effective Intensity

**IES** Guide for Calculating the Effective Intensity of Flashing Signal Lights (1964) - B-R-Douglas
**IALA standard**: “Recommendations on the determination of the luminous intensity of a marine aid-to-navigation light” (1977)
-     - B-R, FF, or Allard
**ITE** Purchase Specification for Flashing and steady-burn Warning Lights (under revision) - B-R
**SAE ARP5029**  Measurement Procedures for Strobe Anticollision Lights (1999) - B-R
**ECE Regulation No. 65** Uniform Provisions Concerning the Approval of Special Warning Lights for Motor Vehicles - FF
**ASTM** standard on photometry of flashing lights - _being developed by E12.11.WG05 _- to recommend one best method
**CIE **recommendation on photometry of flashing light- _being developed by TC2-49_ - to make international standardization

## A problem with Form Factor Method

- Form Factor method clearly fails for this pulse.

- Raised by D. Couzin at CIE TC2-49 Meeting in April 2000 in London.

- Allard and B-R do not have this problem.
- Allard formula has a physiological basis (convolution with a simplified visual impulse response function).
- Decided to re-examine the existing three methods.

## Analysis on the Conventional Three Methods

- To evaluate the differences between the three conventional methods, computation analyses were made by calculating the effective intensity of 10 different waveforms of pulses with duration from 0.001 s to 100 s.

*I*max

- *T*

- *T*

- *T*

- Duration: *T*

## Results 1

## Results 2

## Results 4

## Results 5

## Results 7

## Ratio should be >1

- Blondel-Rey-Douglas failing.

## Results 8

- Form Factor failing.

## Results 9

## BRD and FF are failing.

- Ratio should be ≈ 1

## Summary of Comparison

- Form Factor method fails for a pulse with a sharp peak added on.
- Both Form Factor and Blondel-Rey-Douglas do not work well for a train of short pulses.
- Blondel-Rey-Douglas do not work well for modulated pulses.
- Allard method does not show problems above.
- Allard results deviate significantly from Blondel-Rey results (higher by ~30 % for rectangular pulses in the 0.1 to 1 s region).

**Summary of Comparison**

## Modification of Allard Method

**Observation:**
- Blondel-Rey equation is believed to be accurate for rectangular pulses.

- Modify the *q*(*t*) function so that Allard results match Blondel-Rey for rectangular pulses, but _can be realized with simple analog circuitry_.

**Approach:**

## Modification of Allard Method

- The function *q*(t) is approximated using two exponential functions:

- where

- Analytical calculation revealed a *q*(*t*) that perfectly matches Blondel-Rey for rectangular pulses:

- ;

- This function, however, cannot be realized with simple analog circuits.

*a*1=0.113, *w*1=0.5
*a*2=0.869, *w*2=0.5

- Example:

**Modification of Allard Method**

## Circuit Example for Modified Allard Method

*C*1*R*1= *a*1
*C*2*R*2= *a*2

## Results 1

**Results** 1

## Results 4

**Modified Allard Method**

**Results** 4

## Results 5

**Modified Allard Method**

**Results** 5

## Results 9

**Modified Allard Method**

**Results** 9

## Ratio should be ≈ 1

## Slide 30

## Conclusions

- Both Blondel-Rey and Form Factor methods are shown to fail for certain forms of pulses including a train of short pulses.
- Modified Allard method has been developed, with a modified visual impulse response function *q*(*t*) that produces equivalent results to Blondel-Rey for rectangular pulses.
- Modified Allard method gives reasonable results for all forms of pulses including a train of pulses at any duration.
- Modified Allard method can be realized with simple analog circuits for hand-held photometers.
- This method is considered for standardization by ASTM E12, and to be discussed at CIE TC2-49.  Experimental verification is needed.

## CIE Div.1 & Div.2 meetings:  Aug. 24 - 28

-  CIE Division 2 website:   http://cie2.nist.gov

- CIE Div.1 & Div.2 Joint Symposium on
**Temporal and Spatial Aspects of Light and Colour Perception and Measurement**
- Veszprem, Hungary, **Aug. 22-23**, 2002

- ASTM E12.11.05 (Flashing lights)

- http://cie2.nist.gov/ASTM_E12_WG05/home.html

## Slide 33

## Results 3

## Results 10

## Circuit Example 2 for Modified Allard Method

**Circuit Example 2 for Modified Allard Method**

## Ratio should be >1

## Results 7

**Modified Allard Method**

**Results** 7

## Results 6