Delivered Light Output - Color Kinetics

1 Evaluating Light OutputTechnical BriefUsing Delivered Light to Specify and Compare LED Lighting FixturesEvaluating Light Output Technical Brief2 Evaluating Light Output : The Importance of Delivered LightLight Output is an informal term for how much Light a fixture produces, and how the fixture emits and distributes that Light . The formal term for data describing the quantity and distribution of the visible Light produced by a Light source or fixture is is common practice for lighting fixture manufacturers to report key photometrics in fixtures specification sheets or data sheets.

2 These usually include basic charts and graphs that describe the power or intensity of a lamp or Light fixture, the way it distributes Light in space or over an area, and its energy efficiency. Lighting specifiers and designers use this data to make preliminary evaluations of a lighting fixture s capabilities and its potential suitability for a particular task or application. Fixture manufacturers often make more detailed information available for deeper analysis, creating computer renderings, building mock-ups, and so on.

3 The specification most commonly used for evaluating and comparing the performance of conventional lighting fixtures is lumen Output . Technically speaking, the lumen is a standard unit that measures the total perceived power of a Light source. (Exactly what total perceived power of a Light source means is discussed in more detail on page 5.) In practice, lighting professionals, purchasers, and users often refer casually to a fixture s brightness. This is not only inaccurate, but it can be misleading, especially where LED lighting fixtures are Luminaire testing labs report raw photometric data in.

4 IES files, the format of which conforms to standards set by the Illuminating Engineering Society of North America (IES). Reputable fixture manufacturers make these files available to lighting designers, who use them with lighting analysis software tools to create charts, graphs, and dimensional renderings, and to make comparisons between fixture types. Evaluating Light Output Technical Brief3 For a number of reasons, lumen Output is not the best measurement of an LED lighting fixture s capabilities.

5 In fact, evaluating an LED lighting fixture solely or primarily on the basis of its lumen Output can underestimate or otherwise give a distorted picture of the fixture s performance and suitability for a given task or of lumen Output , the best and most relevant measurement for evaluating LED lighting fixtures and for making accurate comparisons with conventional lighting fixtures is Delivered Light . The formal term for measurements of Delivered Light is illuminance. Roughly speaking, illuminance is the intensity of Light falling on a surface area.

6 If the area is measured in square feet, the unit of illuminance is footcandles (fc). If measured in square meters, the unit of illuminance is lux (lx). Delivered Light describes how much useful Light a lighting fixture can deliver to a task area. Useful Light is the portion of a lighting fixture s Light Output that is effectively directed to a task area, discounting any wasted Light . The task area can be any space or surface that requires illumination an entrance hallway, a common office space with desktop computers, a kitchen countertop, or the face of a Mayan pyramid in Guatemala.

7 Light can be wasted in a number of ways: It can be partially blocked or dispersed within the fixture housing, it can be emitted in a direction away from the task area, or it can be lost through filtering, lensing, fixture positioning, or any of a number of other factors relevant to a specific installation. A look through The IESNA Lighting Handbook, the 1,000-page standard reference work from the Illuminating Engineering Society of North America (IES), demonstrates the importance of Delivered Light as a lighting principle, especially for white- Light and everyday applications.

8 Along with chapters that describe in detail how to deliver the right amount of useful Light in an application, the Handbook includes an extensive lighting design guide that specifies ideal Light levels for every conceivable interior, industrial, outdoor, sports, transportation, and emergency lighting example, the Handbook recommends a Delivered Light level of around 30 fc (300 lx) for an open plan office with extensive computer use, as well as for the ticket counter of a transportation terminal.

9 Freight elevators should have a level of around 5 fc (50 lx), while serious reading in a chair at home requires a level of around 50 fc (500 lx). Lecture halls where demonstrations are being performed should have a Light level of around 100 fc (1000 lx).A computer graphic ray trace image, rendered in AGi32 lighting analysis software, from raw photometric data for ColorBlast Powercore LED fixtures from Philips Color Kinetics . The image models ColorBlast Powercore lighting effects in an exterior glass : Chris Fournier and Matt PayetteE Technically.

10 , there are lux per footcandle. However, to simplify the math, lux measurements are typically given as footcandles x 10. 30 fc is therefore equivalent to 321 lx, but is expressed as 300 lx by convention. Evaluating Light Output Technical Brief4 The Trouble with LumensThe way that lumen Output is traditionally measured, reported, and interpreted poses a number of potential problems for evaluating and comparing LED lighting fixtures:Since complete and accurate definitions of lumens and related photometric terms can be technical and complex, they are often misunderstood.