Producing photometric data – what do we need?


Specification of data required for a luminaire – photometric data


There is a practical reason for data collection on luminaire performance to be standardised. With BIM (Building Information Modelling) becoming a design process norm and specialist specification companies like Spektd coming to market, manufacturers will need to respond in a positive way.

The Light Review asked Richard Hayes to set out the criteria for photometric testing of luminaires.

7 Steps to Good Lighting
SPECIFICATION

Intensity distribution

A full intensity distribution, presented as candela (cd) or candela per kilo lumen (cd/klm), is required for each luminaire. Angular steps of intensity data should be suitable and sufficient to allow accurate calculations of luminaire luminance and resultant illumination to be made.

Table 1 data step requirements.

Beam WidthAngular Data Step
<10° 0.5°
10° to 20°
20° – 30° 2.5°
>30°

Note 1:
Luminaires may have different beam widths in different planes, photometric intensity distributions can have varying data steps. These figures apply to the width of any beam in any plane.

LED luminaires can produce jagged distributions, if a relatively smooth distribution has a sudden jagged peak, this part of the distribution should be treated as a “beam” and angular steps applied accordingly.

Goniophotometer systems for intensity measurement must maintain adequate temperature stability. Goniophotometers that move or change the orientation of the luminaire under test must be able to demonstrate stability of the luminaire under test during any movement or orientation change. All goniophotometer systems have limits on measurement angles and have some reflection components, for raw data uncertainty of measurement must be stated, manufacturers should declare corrections made to correct raw data for practical purposes.


Measured data and practical data.

Raw Goniophotometer data may need correction factors and measurements applied to reduce the effects of limitations of the Gonio system in order to produce practical data with correct cut off angles and to complete all angles of data to produce practical photometric data for calculation purposes.

Most luminaires have some symmetry, for instance a rectangular down light may have 2 planes of symmetry, one plane through the long Transverse axis (0°-180° azimuth) and one plane through the short Axial axis (90°-270°) azimuth. In this case one quadrant of data 0°-90° repeated 4 times can describe the whole luminaire. Rotationally symmetric luminaires (round downlights, round spotlights) can be described by one plane of data which can be repeated at all azimuth angles.

When carrying out a photometric test on a real luminaire there will always be some degree of difference between planes intended to be symmetric, this will be due to manufacturing tolerances,  provided this is not excessive, for practical application the planes of symmetry should be averaged and the averaged data file used for calculation purposes. Commonly, streetlights and floodlights will have one vertical plane of symmetry.


Lumen output

Measurement of Lumen Output can be made using a goniophotometer or photometric integrator, in each case the system must be temperature stable. Generally it is easier to maintain close temperature control in an enclosed integrator than in a large Goniophotometer chamber. Goniophotometers that move or change the orientation of the luminaire under test must be able to demonstrate stability of the luminaire under test during any movement or orientation change. Integrators must be properly calibrated and have coatings that can demonstrate high and uniform reflectance across the visible spectrum.


Spectral Power Distribution.

SPD can be measured using either Goniophotometer or Integrator systems, Spectral data can be processed to produce photopic output and other output systems such as Scotopic output or Melanopic output.


Consistency

For any complete set of photometric data the intensity data integrated against the V/Lamba curve must be equal to the measured lumen output. Computer lighting design programs will provide warnings or change values (usually Lumen output) if this is not the case. Any data where this occurs is in error.


Dimensions

Accurate dimensional data is important, derived data such as luminance and UGR values depend on accurate dimensional information. For computer use EULUMDAT(ldt or ldc) file format is preferred. EULUMDAT enables definition of both Luminous and overall dimensions. IES file format allows only definition of Luminous dimensions so should be treated with caution, it will be impossible to determine the overall size of a luminaire from an IES file. If IES files are supplied it is vital they are correctly formatted and that dimensions refer to luminous dimensions, if overall dimensions are included in an IES file the derived data will be in error.


Power

Power for luminaires must be total circuit power at the point of supply, all driver, ballast and parasitic power (photocells, sensors, communication systems) must be included in the measured power.


Traceability.

Data must be traceable. In practice a family of luminaires may have a common photometric distribution, family members may vary due to power or colour of a common format of LED, or may have different lengths of a common reflector or refractor system. In this case manufacturers should make it clear that the data uses a common distribution and that measurements of varying Lumen output and power have been made on the variant family members. Manufacturers must be able to supply traceablity of all data on request. Ideally raw measured data will be available as well as practical symmetric data.

But what about manufacturing quality: the CE Mark?

The Lighting Industry Association (LIA) has produced a Technical Statement on this topic:

LIA TS32: ESSENTIAL REQUIREMENTS FOR LIGHTING PRODUCTS UNDER THE CE MARK
(Issue 3: March 2017)

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