How to Read a Photometric Report
The introduction of computer aided programs has made the possibility of lighting design an everyday event for building services engineers and designers of lighting installations. Lighting design is no longer the sole province of the lighting design specialist, since any interested person with some basic knowledge of Lighting can, with the aid of a computer, experiment with ideas.
However as in all areas associated with computer practice the old saying of “Garbage in, garbage out” still applies, and not all lighting design presented from a computer print out should be believed. Several very important factors must be taken into consideration when using computer aided lighting design programs, and these are discussed below.
However as in all areas associated with computer practice the old saying of “Garbage in, garbage out” still applies, and not all lighting design presented from a computer print out should be believed. Several very important factors must be taken into consideration when using computer aided lighting design programs, and these are discussed below.
Quality of Photometric Data
One of the primary data inputs to a lighting design program is the photometric data, and if this data has not been obtained by a very high quality of testing procedure of the luminaire, then it is certainly not worth including in the design process. Similarly the matter of what format is used for the transition of the electronic data must be carefully considered, since it may or may not be compatible with the particular design program being used. To ensure that the quality of the testing procedure is both of a very high standard and that the results from various laboratories are comparable, these two matters are addressed in Australian Standard AS1680.
Standards Australia has researched and published AS1680.3 - “Interior Lighting, Part 3 Measurement, Calculation and Presentation of Photometric Data.” This Australian Standard closely follows the procedures set down in the CIE Publication No. 24 (TC 2.4), and so there is a full agreement between the Australian practice procedures and that of CIE international practice on the presentation of photometric data.
Standards Australia has researched and published AS1680.3 - “Interior Lighting, Part 3 Measurement, Calculation and Presentation of Photometric Data.” This Australian Standard closely follows the procedures set down in the CIE Publication No. 24 (TC 2.4), and so there is a full agreement between the Australian practice procedures and that of CIE international practice on the presentation of photometric data.
Presentation of Luminaire Intensity Data
It is accepted practice in this and many other countries to use the CIE CGamma (Cg) co-ordinate system for the specification of Intensity data for indoor commercial and industrial luminaires and street lighting lanterns. (Figure 1). The number and spacing of the azimuth planes (C planes) and the vertical angles (gamma angles) largely depend upon the accepted practice of the individual laboratories.
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Components of a Photometric Test Report
Test reports present a large amount of information in a number of different forms to the user. LightLab reports include a Summary page which yields a quick, concise overview of the data at a glance. See Figure 2 for a sample of the Summary page.
The text includes all the descriptive information in the Test Report which is not measured or calculated. Never the less, it is just as important to the validity of the report as the numerical content. The text includes a description of the luminaire. Other text components of a report are the certification including the signature of the person certifying the authenticity of the report, the dating of the document and the numbering of all pages.
The primary information in the header is who is the manufacturer, and the catalogue number of the luminaire tested. The description of the luminaire describes the body and its overall dimensions, how the control gear is mounted, and how the luminaire itself is to be mounted, i.e. recessed or surface mounted. Also described are the luminaire components, including whether the inner reflecting surfaces are painted or powder coated and the nature of reflectors and louvre if used or the plastic light control panel such as a prismatic inlay. This information is provided so that the luminaire can be easily identified. A certified Test Report also carries a document reference number This should also be checked against manufacturer’s published information. |
The make and type of lamps and ballasts with their appropriate identifications are also included, as are the luminous opening dimensions. In the case of emergency luminaires tested to AS/NZS 2293 a diagram showing which lamp is activated may also be included. The luminous opening data is used to calculate the average luminance values (units are cd/sq.m/klm) and this is shown in a separate table on the Summary Page.
Other important textual information is the signing of the report. If the report includes an official NATA emblem to show that it is an Endorsed Document, then the signatory must be a person accredited by NATA. While this person is signing on behalf of the testing laboratory, it is that person who is responsible for the validity of the document.
The report must also be dated at the time of signing. This is important because it is generally accepted that the life of a report is about five years. This is not because the report itself is outdated, though Standards may have changed in that time, but because it is highly unlikely that the manufacturer will produce exactly the same luminaire with the same components five years later.
Each page is numbered (e.g. “Page 1 of 5”, “Page 2 of 5” etc.). This is important because the document is only valid as a NATA certified report if it is reproduced in full. Where a partial or incomplete reproduction of a report is offered, one must wonder why was the rest of the report was omitted.
The last line of the Summary page carries the name of the person or company for whom the report was prepared and who commissioned it. While that person or company is free to distribute the report (in full) to whom they wish, the bond of confidentiality held between LightLab and its client demands that LightLab can only discuss the contents of the report with a representative of the client company, unless the express approval of the client is received.
Other important textual information is the signing of the report. If the report includes an official NATA emblem to show that it is an Endorsed Document, then the signatory must be a person accredited by NATA. While this person is signing on behalf of the testing laboratory, it is that person who is responsible for the validity of the document.
The report must also be dated at the time of signing. This is important because it is generally accepted that the life of a report is about five years. This is not because the report itself is outdated, though Standards may have changed in that time, but because it is highly unlikely that the manufacturer will produce exactly the same luminaire with the same components five years later.
Each page is numbered (e.g. “Page 1 of 5”, “Page 2 of 5” etc.). This is important because the document is only valid as a NATA certified report if it is reproduced in full. Where a partial or incomplete reproduction of a report is offered, one must wonder why was the rest of the report was omitted.
The last line of the Summary page carries the name of the person or company for whom the report was prepared and who commissioned it. While that person or company is free to distribute the report (in full) to whom they wish, the bond of confidentiality held between LightLab and its client demands that LightLab can only discuss the contents of the report with a representative of the client company, unless the express approval of the client is received.
Measurement Data
The fundamental measured data consists of a photometric intensity distribution, comprising the directional luminous intensity values. Typically the intensity data is expressed in absolute candela, as most modern luminaires are designed with integral lamps. In some cases, the report may be relative to the luminous flux of the installed lamps, in which case the photometric data is expressed as candela per 1000 lamp lumens (cd / klm). The relative format allows lamps with a common form factor to be interchanged by assigning the luminous flux of the target lamp.
Measured data is presented in photometric reports in graphical and tabular form (see Figure. 4). The graph is usually in the form of a polar diagram, expressed using the Cγ coordinate system where the γ angles represent the altitude and the C plane represent the azimuth. Data from a number of different C planes may be plotted on the graph.
The table of luminous intensities can be used for making quick checks and comparisons between different luminaires. This data is also offered to the client in the form of an “IES” format file for use with PC based computer design programs. |
Another part of a laboratory photometric report includes derived data, and this is in general as required by the Australian Standard AS1680.3 “Derived Luminaire Data.”
The major derived values for a luminaire are the Utilisation Factors, the Spacing to Mounting Height Ratios, Average Luminances, Zonal Flux and the Light Output Ratio. The determination of the Light Output Ratio (LOR) is usually in accordance with Appendix A of AS1680.3.
The major derived values for a luminaire are the Utilisation Factors, the Spacing to Mounting Height Ratios, Average Luminances, Zonal Flux and the Light Output Ratio. The determination of the Light Output Ratio (LOR) is usually in accordance with Appendix A of AS1680.3.
Flux basis
Typically, the flux basis is either absolute or relative, with absolute being more prevalent with the introduction of LED lighting where the light sources in the lamp are typically not designed to be end-user replaceable. An absolute report expresses the luminous flux as lumen (lm). A relative report may, much like the reporting of the luminous intensity, report the luminous flux relative to the luminous flux produced by the lamps, the ratio of the two is Light Output Ratio (LOR).
Generally there are three primary factors which determine the LOR: (a) the light absorption within the luminaire, (b) the change in light output of the lamps due to the thermal conditions within the luminaire, and (c) the difference between the lamp power delivered by the normal proprietary ballasts and the laboratory reference ballast.
If only factors (a) and (b) are considered, the result is LORL or the “Light Output Ratio Luminaire. This is defined as the ratio of the LOR of the luminaire with normal ballasts, as compared to the total light output of the lamps using the same ballasts, but operating outside the luminaire under reference conditions.
When all three factors are considered the result is LORW or the “Light Output Ratio Working.” This is the ratio of the light output of the luminaire operating under specific conditions with reference ballasts, as compared with the total light output of the lamps, operating outside the luminaire, under reference conditions with the same reference ballasts.
There is no international agreement on a standard method of determining LOR, and as a general rule the LORL is the parameter which is used in Australian relative photometric reports.
Generally there are three primary factors which determine the LOR: (a) the light absorption within the luminaire, (b) the change in light output of the lamps due to the thermal conditions within the luminaire, and (c) the difference between the lamp power delivered by the normal proprietary ballasts and the laboratory reference ballast.
If only factors (a) and (b) are considered, the result is LORL or the “Light Output Ratio Luminaire. This is defined as the ratio of the LOR of the luminaire with normal ballasts, as compared to the total light output of the lamps using the same ballasts, but operating outside the luminaire under reference conditions.
When all three factors are considered the result is LORW or the “Light Output Ratio Working.” This is the ratio of the light output of the luminaire operating under specific conditions with reference ballasts, as compared with the total light output of the lamps, operating outside the luminaire, under reference conditions with the same reference ballasts.
There is no international agreement on a standard method of determining LOR, and as a general rule the LORL is the parameter which is used in Australian relative photometric reports.
Zonal Flux
The Summary Page includes a table which shows (in this case a relative report) the luminous flux per light source klm in a number of zones. The table includes percentages of the lamp lumens and luminaire lumens in each of the zones. See Figure 4 for a typical report presentation of zonal lumens. The sum total of the lumen and percentage lamp columns equates to the overall light output ratio of the luminaire tested.
These zonal lumen values are very useful when comparing two luminaires, which although they may have the same LOR can actually have a totally different light distribution. One may have all its light output concentrated in a very narrow zone i.e. 0° - 30°, whereas the other may have a significant proportion of its light in the 60° - 90° zone. These two luminaires will create a totally different visual environment, even though they have the same LOR. |
Spacing to Mounting Height Ratio
The CIBSE Publication TM 5 also presents the proposed methods for the determination and presentation of Spacing to Mounting Height Ratios (SHRs) as shown in AS1680.3. The Australian Standard AS1680 series 2 Standards include recommendations on preferred uniformity ratios for specific tasks or interiors. Uniformity is defined as the ratio of the minimum illuminance, compared with the average illuminance in the space.
Uniformity relates the ratio of the luminaire spacing to the mounting height of the luminaires. Generally there are two values given. A nominal spacing to mounting height ratio (SHR NOM) and a maximum value (SHR MAX). (Refer Figure 5)
Average luminance
Generally for bi-symmetric interior lighting luminaires the Summary page includes a table of average luminance values (cd/sq.m/klm) in the zones from 45° to 85°. See Figure 7 for a typical average luminance table.
This is included because Australian Standard AS1680.1, “Interior Lighting Part 1: General Principles and Recommendations, Section 8, “Glare and Related Effects” includes a luminance limiting system as a means of limiting the degree of discomfort experienced by the occupants of a space from glare created by ceiling mounted luminaires. Tables 8.2 and 8.3 of AS1680.1 set out a scale of maximum average luminance values for various tasks and interiors. The luminance values quoted on the Summary page can, providing the correct total lamp lumens per klm multiplier is used, be equated with the limits set in Table 8.2 and 8.3 of the Standard. |
Utilisation Factors
t is common usage in Australia to adopt the method for the derivation and presentation of Utilisation Factors in accordance with the British CIBSE Publication TM 5.
Appendix C of the AS1680.1 sets out assumptions and limitations which apply to the use of utilisation factors in the design of interior lighting. Utilisation factors are used to define the effectiveness of a luminaire in a particular space. They are included in the report for use in the “Lumen Method” used to approximate the number of luminaires required in a particular space. Figure 8 shows a representation of a Utilisation Factors table from a typical report. |
Uncertainties and measurement methodology
The final page of the test report states the conditions under which the test was conducted. For example, the test distance, test temperature, testing procedure including the applicable standards or recommended international practices. The uncertainties of measurement are also included. The estimation of uncertainties is a very complex subject, with many books offered on the subject.
LightLab include uncertainties of measurement of temperature, light output ratio, luminous intensity and angular displacement. There are other uncertainties which may be included such as physical dimensions and electrical parameters.The particular uncertainties quoted on the final page of reports are the result of the laboratory’s careful assessment of the performance of each piece of equipment or system used to obtain the values reported. NATA assessors examine the laboratory’s equipment and documentation to check the validity of the claimed uncertainties. The laboratory staff is required to carry out regular calibration checks to ensure that the equipment is within the range of uncertainties which are claimed.
LightLab include uncertainties of measurement of temperature, light output ratio, luminous intensity and angular displacement. There are other uncertainties which may be included such as physical dimensions and electrical parameters.The particular uncertainties quoted on the final page of reports are the result of the laboratory’s careful assessment of the performance of each piece of equipment or system used to obtain the values reported. NATA assessors examine the laboratory’s equipment and documentation to check the validity of the claimed uncertainties. The laboratory staff is required to carry out regular calibration checks to ensure that the equipment is within the range of uncertainties which are claimed.