Bilancio delle perdite ottiche

The illuminance a luminaire delivers on the day it is installed is a peak, not a design value. Every lighting installation loses light from the moment it is energised, through dirt on the luminaire and the room surfaces, through the slow decay of the LEDs, through the ageing of the driver and the optical materials, and through the unavoidable losses in the optical system itself. The light loss factor budget is the disciplined accounting of those losses, and it is the difference between a design that still meets its target after five years and one that never met it after commissioning. This white paper sets out how to build that budget for industrial, cleanroom and tri-proof installations, which factors dominate, and how to verify the assumptions against real data.

The reader is a lighting designer, a project engineer or a procurement engineer who has to justify a fixture count that looks higher than a competitor’s, and who needs the arithmetic to do it.

1. The two distinct loss categories

Light loss factors divide into two groups that behave completely differently, and conflating them is the source of most budget errors.

Only the recoverable factors can be restored by maintenance, and only the recoverable factors can justify a lower design factor in an installation that is genuinely well maintained. A cleaning regime that is promised but not resourced does not reduce the dirt factor; it merely moves the shortfall from the design stage to the complaint stage.

2. Optical efficiency: where the light goes

Optical efficiency is the fraction of the flux emitted by the LED packages that leaves the luminaire aperture within the design beam. Losses arise from four mechanisms.

The waterfall demonstrates the central point of this white paper: the maintained value is roughly a quarter of the LED package flux in a demanding industrial environment, and roughly a half in a benign one. A design performed on LED flux therefore overstates the delivered illuminance by a factor of two to four.

The four optical mechanisms account for the first three transitions:

• Reflection and transmission loss. A reflector with 92 percent reflectance used in a single-bounce geometry passes 92 percent of the incident light, but a double-bounce geometry in a deep cell passes only 85 percent. Diffusers transmit between 70 and 90 percent depending on the diffusion grade and thickness.
• Blockage and absorption. The edges of the optic, the heat sink, the driver housing and the gaskets mask part of the emitted flux. In a compact high-bay this is typically five to ten percent.
• Back-scatter. Light that re-enters the optic and is absorbed rather than transmitted. Small in a well-designed optic, substantial in a poor one.
• Spectral shift. Not a loss of flux but a change in its spectral composition, which matters where the luminaire is specified for a spectral target, as in horticultural and colour-critical applications.

The last two rows illustrate a constraint that is frequently overlooked at the concept stage: sealing a luminaire against water and dust, or against particulate contamination in a cleanroom, costs optical efficiency. The optical path must pass through a gasketed interface and often through a secondary protective sheet, and each interface loses two to eight percent. A luminaire designed for IP65 cannot match the optical efficiency of an unsealed indoor luminaire with the same LED package, and a specification that demands both the highest efficacy on the market and an IP65 rating is internally inconsistent.

3. Lumen depreciation and the TM-21 projection

Lumen depreciation is the decline in LED flux over time. It is characterised by the LM-80 test, which measures luminous flux maintenance at defined drive currents and case temperatures for a minimum of 6,000 hours, and projected by TM-21 to a longer period using an exponential model.

The dependence on case temperature is the important engineering content of the table: a ten degree reduction in case temperature extends the projected life by roughly thirty percent. This is why the thermal design of a luminaire matters to the light loss factor budget, not only to reliability. A luminaire with a better heat sink can be designed with a lower lumen depreciation factor, which reduces the required fixture count, which reduces the installed load — a compounding benefit that is invisible in a datasheet comparison of efficacy alone.

TM-21 projection has defined limits, and exceeding them invalidates the projection. The rule is that the projection period may not exceed six times the LM-80 test duration, and the projected value must be reported with the case temperature at which it applies. A claim of “L70 at 100,000 hours” without a stated case temperature and an LM-80 duration sufficient to support it is not a TM-21 projection in the standard’s sense.

The choice between L70, L80 and L90 as the design basis is a commercial decision, and it must be stated. Designing to L70 means that at the end of the design life the installation delivers seventy percent of initial flux, which may be well below the required maintained illuminance. Most well-run projects design to a stated maintenance factor derived from the flux at the design hour rather than to an L-value, and report the L-value as supporting data.

4. Dirt depreciation

Dirt depreciation has two components, and they are frequently merged incorrectly.

The combined factor is the product, and it depends on three environmental inputs and one maintenance input.

The range in the final column, from 0.48 in a mine to 0.93 in a cleanroom, is the largest single factor in the budget. It is also the factor most likely to be assumed rather than derived: a designer who applies a default of 0.80 to a mining installation will under-provision the fixture count by a third, and a designer who applies 0.80 to a cleanroom will over-provision it by nearly fifteen percent.

5. Maintained illuminance over time

The combined effect of lumen depreciation and dirt accumulation is a falling illuminance curve, and the shape of that curve determines when cleaning and when replacement become necessary.

The saw-tooth curve is the one that matters commercially, because it shows that cleaning extends the period during which the installation meets its requirement more effectively than any other single intervention. In this example the cleaned installation remains above the 75 percent requirement for the whole sixty-month window, while the uncleaned installation in the same dusty environment drops below it at about month fourteen. The cost of the annual cleaning programme is small relative to the cost of a retrofit that would otherwise be required at month fourteen.

Two cautionary notes on this interpretation. First, the recovery from cleaning is partial: cleaning restores the dirt component but does not restore the lumen depreciation component, so each saw-tooth peak is lower than the last. Second, cleaning carries its own risk of damage to the diffuser surface, and a badly executed cleaning programme can reduce optical efficiency permanently through abrasion.

6. Worked budget for three installations

The tables below build the full budget for the three product families, using environment-specific factors.

6.1 Cleanroom installation — Z-13

6.2 Tri-proof installation — Z-15

6.3 High-bay installation — Z-16

The three factors span a wide range — 0.42 to 0.65 — for luminaires of comparable LED quality, and the spread is driven almost entirely by the sealing requirement and the environment rather than by the LED package. This is the single most useful insight for procurement: when comparing two suppliers’ fixture counts for the same space, check the light loss factor each has assumed before comparing the number of luminaires.

7. Common budgeting errors

8. Verification

The light loss factor budget is verified in two stages: once at design, against the manufacturer’s data, and once in service, against measurement.

The last two checks are rarely performed and they are the ones that close the loop on the largest factor in the budget. A single luminaire measured immediately before and after cleaning gives a direct LDD value for that environment, and three such measurements over two years validate or refute the assumed cleaning interval for an essentially trivial cost.

9. Conclusion

The light loss factor budget is arithmetic, and the arithmetic is not difficult. What makes it difficult in practice is the discipline of deriving each factor from the actual environment rather than accepting a default, and the discipline of documenting the basis of each factor so that the design can be defended and verified.

The working sequence for an industrial or cleanroom project is: start the budget at the luminaire aperture rather than the LED package, take the optical efficiency from the measured photometric file, take the lumen depreciation from an LM-80 report with a stated case temperature and a TM-21 projection within the permitted period, derive the dirt factors from the environment class and the resourced cleaning interval, and then verify the initial and year-one illuminance by measurement.

For a Z-16 high-bay installation in a general warehouse, the combined factor is typically around 0.55, dominated by the dirt component rather than the LED. For a Z-15 tri-proof installation in a wash-down food environment, the factor can fall to about 0.42, driven by both the sealing penalty on optical efficiency and the aggressive dirt environment. For a Z-13 cleanroom installation, the factor is the most favourable of the three at around 0.65, because the environment that demands the sealed diffuser is also the environment that keeps it clean. Designing to these factors rather than to nominal flux is what makes the difference between a compliant installation and a retrofit at year three.

10. Referenced standards

• IES LM-79 — Approved method: electrical and photometric measurements of solid-state lighting products
• IES LM-80 — Approved method: measuring luminous flux and colour maintenance of LED light sources
• IES TM-21 — Projecting long term lumen maintenance of LED light sources
• CIE 97 — Maintenance of indoor electric lighting systems
• CIE 154 — The maintenance of outdoor lighting systems
• EN 12464-1 — Light and lighting: lighting of work places, indoor
• IEC 60529 — Degrees of protection provided by enclosures (IP code)
• GB 50034 — Standard for lighting design of buildings

11. Contact us

QUEENDOM supplies light loss factor documentation for the industrial, cleanroom and tri-proof ranges, including the photometric files behind the optical efficiency figures and the LM-80 and TM-21 reports behind the lumen depreciation projections for the Z-13, Z-15 and Z-16 families. Provide the room dimensions, the environment class, the intended cleaning interval and the design life, and the engineering group will return a completed light loss factor budget, the maintained illuminance calculation and the fixture schedule derived from it.

Light-Loss Factor Budget and Maintained-Illuminance Decay

The light-loss factor is a product of independent multipliers, and its weakness is that losses compound silently. The first curve decomposes a typical budget so each contribution is visible; the second projects maintained illuminance over the service interval to show where the design floor actually lands.

Figure 1 – Cumulative light-loss factor as each contributor is applied. Representative budget.

In this representative budget the luminaire dirt depreciation and the LED lumen-maintenance term together account for over half the total loss. That is a useful ratio: it says the highest-return interventions are dust ingress control and choosing a higher-maintenance L90 bin, not tightening the initial lumen figure.

Figure 2 – Maintained illuminance index over a ten-year service interval against a 0.70 design floor. Representative projection.

With an LLF of 0.72 the maintained illuminance at year 10 sits about 28 % below the initial value. A layout dimensioned on initial lumens alone will therefore be under-lit for most of its service life — the reason maintained illuminance, not initial, is the number a compliance check should use. Our industrial and commercial ranges publish the input terms so this budget can be reproduced.

Related products and applications

The luminaire families used in the light-loss budget are listed below.

• Industrial high-bay luminaire (Z-13)
• Tri-proof luminaire (Z-15)
• High-bay luminaire (Z-16)
• Application overview: Lighting application solutions
• More technical papers: Lighting knowledge resources