Interpreting Point-by-Point Illuminance Predictions
Photometry and Light Level Predictions
The quantitative results of a new or retrofit lighting system can, and should be well understood prior to purchase and installation. The photometric measurement and calculational tools necessary to predict light levels in application are well established and commercially available. These methods are accurate and reliable when practiced by qualified lighting professionals, and are generally the primary basis upon which product performance is evaluated throughout the commercial construction industry.
The laboratory procedures and measurements for determining the light output characteristics of a luminaire are called photometry. Photometric measurements are taken in a large black room with a specially designed and calibrated device know as a goniophotometer. Measurements are taken under controlled thermal and electrical conditions with a production version of a luminaire equipped with its intended light source (lamp). A multitude of light readings are then taken about a spherical web surrounding the luminaire to quantify the intensity of light emitted in every direction. This characterizes the photometric distribution of the product. This data also captures the total amount of light leaving the luminaire, which determines its photometric efficiency – the ratio of the total light output of the luminaire to the light output of the lamp (or lamps) within.
The most common photometric data format in the U.S. is commonly referred to as an IES file. This format is governed by the Illumination Engineering Society of North America (IESNA) and usually has a .ies file extension when utilized as an electronic computer file. An IES file for a particular luminaire can be used as input to lighting design software where the encoded efficiency and distribution information is used to predict the net effect of a system of luminaires in a specific application. This software can be used to model the elements of an application that affect the propagation of light (mounting location, room geometry, occlusions, surface reflectances, etc…) and to predict the resulting light levels anywhere within the space. Predicted light levels are typically conveyed in the form of illuminance values associated with a planar grid of points that may or may not coincide with an actual physical surface. This is commonly known as a point-by-point analysis and the plane in which the grid resides is referred to as the workplane. It may correspond to a real physical surface such as a floor or wall, or it might simply represent the location and orientation of important visual tasks (such as the height above the floor at which assembly operations are typically performed or the face of a storage rack where cartons are identified for picking).
In addition to the predicted illuminance at individual point locations on a surface, a point-by-point analysis should also include statistical metrics such as the average, minimum and maximum illuminance as well as the maximum-to-minimum and the average-to-minimum illuminance ratio. These measures are useful for quickly gauging the overall quantity as well as the uniformity of illuminance in a specific area.
Light Loss Factors and Other Sources of Error
As with all computer modeling, the reliability of point-by-point predictions is highly dependent upon the accuracy and appropriateness of the input. Common sources of error include using photometry specific to a different version of the luminaire (such as with a different lamp type), the accuracy of room surface reflectances, and whether or not significant light blocking objects (such as storage racks, partition walls and large pieces of equipment) were accounted for in the model of the application space. Also of particular importance is to ensure that the total luminaire count is correct and also that the modeled luminaire mounting heights and spacings between luminaires match with the intended design.
Another major source of error and confusion involves what is known as a Light Loss Factor (LLF). The light output (measured in lumens) of all electric light sources diminishes with age. Additionally, any build up of dust or other airborne contaminants during luminaire maintenance intervals can diminish the efficiency of the optical system. These effects reduce the light available for visual tasks, and should be accounted for with an appropriate Light Loss Factor. The LLF should be composed of at least a Lamp Lumen Depreciation (LLD) factor and a Luminaire Dirt Depreciation (LDD) factor in addition to any other loss mechanisms that are particular to the application. The appropriate Lamp Lumen Depreciation factor is particular to the lamp type and manufacturer as well as the ballast equipment operating it. All of these inputs should come from manufacturer published data based on laboratory measurement. A Luminaire Dirt Depreciation factor of 0.90 is typical for light industrial applications, but should be lower number (to account for greater loss) for heavy industrial applications, applications that are particularly dirty or associated with other airborne contaminants as well as applications where luminaire maintenance is prohibitive. A lower LDD should also be considered for lighting equipment with a significant portion of the lamp(s) or optical surfaces that are oriented horizontally, and thus prone to a more rapid accumulation of settling dust and other particulates.
Applying an appropriate Light Loss Factor to point-by-point light level predictions gives a much more realistic estimate of system performance over time. Illuminance values accounting for loss mechanisms in this way are typically referred to as “maintained light levels”. An inaccurate or missing Light Loss Factor is a common source of discrepancy between predicted and actual results - but one that is not revealed until long after installation and commissioning (or during a trial installation). This standard procedure of applying an LLF also ensures that competing equipment and design options are put on a level playing field for comparative purposes. It is however also useful to have a sense for expected “initial light levels” when it comes to evaluating a trial installation or during the commissioning phase of a new lighting system.
Interpreting the Numbers
Point-by-points illuminance calculations are most useful for a quantitative evaluation of the average light levels that a given design and set of products will provide. Individual point grids and illuminance statistics should always be calculated and interpreted relative to the location of specific visual tasks and functional areas. These function and task specific illumination levels can then be compared to industry recommended standards (see IES Recommended Levels) or existing light levels in the case of a retrofit scenario. These comparisons should be evaluated in terms of both task visibility and energy usage. Under-lighting can have critical ramifications - error rates, safety, employee and./or customer satisfaction, as well as both perceived and real productivity. Conversely, over-lighting does not generally improve visibility and represents an unnecessary and costly over-use of energy (see Tailored Lighting below for more detail).
As lighting calculations take into account the delivery efficiency and characteristic light distribution pattern of particular equipment as well as the overall count and placement of the luminaires, point-by-points are also very useful in comparing both design and product options.
In addition to the average level of illumination on a surface or workplane, point-by-point calculations can also provide some insight into the uniformity of the illuminance on a surface. Illuminance uniformity is more difficult to interpret, however, than average illuminance levels. Uniformity is typically measured by either an average-to-minimum illuminance ratio or a maximum-to-minimum illuminance ratio. As a general rule of thumb, it is best for a functionally homogeneous area with similar visibility needs to not exceed an average-to-minimum illuminance ratio of around 2:1. However, any negative visual effects associated with non-uniformity are very dependent on the abruptness of the transition (from one level to another) and the nature of the visual task. Transitions that are very smooth can cover a wide range of illuminance without even being visually noticeable. Depending on the situation, a lack of uniformity is not necessarily problematic. In fact, deliberate variations in light level between functionally separate areas can enhance focus and add visual interest and vibrancy to a space as well as save significant energy (see Tailored Lighting for more detail).
Significant ratios between nearby illuminance points may indicate a visual problem due to shadowing, a poor or inappropriate photometric distribution or insufficient luminaire coverage. However, as discussed below, not all visual problems will be readily apparent by looking at point-by-point values.
To be most valuable, grid point values should be spaced densely enough to evaluate the uniformity of illumination everywhere within an area of similar function. Illuminance point grids should also extend far enough or be dense enough to cover areas that naturally tend to be less lit, such as in between luminaires and at the edges of a space.
It is important that the orientation (in addition to the placement) of a point grid be consistent with the visual task or function being considered. For example, if the primary visual task is oriented vertically, such as the identification or manipulation of product on vertical racks, then the illuminance levels should be calculated for all points of interest on the vertical surface.
Finally, it is often appropriate to focus analysis on a typical portion of a particular functional area rather than calculate and convey illuminance values for all possible locations with the same features. For instance, if an assembly area has several identical production lines that will be lit in the same way, then performing detailed calculations for just one line or even a typical section of one line makes the data presentation and evaluation more approachable.
What a Point-by-Point Analysis Doesn’t Tell You
By no means are all the important qualities of a lighting installation captured in a simple point-by-point analysis. Perceptions such as glare, overall brightness, and impressions such as clarity and the “tone” of the visual environment are difficult, or even impossible to glean from point-by-point illuminance predictions. Certain issues such as shadowing and the modeling of three-dimensional objects may also be difficult to assess. There is no substitute for seeing a particular product in application or, when possible, arranging a trial installation (see Evaluating a Trial Installation below for more detail).
Related Resources
Why Lighting is Important
Tailored Lighting
Evaluating a Trial Installation
Glossary of Lighting Terms
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