LED Brightness

LED Lighting- Brightness

LED lighting is a decision that opens up smorgasbord of choice that you don’t have when you opt for halogen or fluorescent lights. Gone are the days of Mum sending you to the nearest shop for a 100 Watt light bulb (And reaching for the nearest domestic weapon when you come back with a 30 watt) Now is the time to familiarise yourself with the technical jargon associated with LED lighting. We will explain terms like ‘Lumen’ and ‘Lux’ to help you make well informed decisions on the kind of LED lighting that will work best for you.

The Lowdown on Lumens, Lux, Watts and Glare

Wattage is the traditional indication of brightness in incandescent lights. Domestic bulbs typically range from 60W-100W.  A 60 Watt light bulb provides sufficient light for reading, but is not bright enough for fine work or cooking in the kitchen. From an environmental (and financial) standpoint, the biggest problem with incandescent lights is that only around 5% is used to create light. The rest of it goes into generating heat, and not the kind that’s going to keep us cosy on those frosty July evenings. It is wasted heat, leaving us with nothing but a cold, hard power bill at the end of the month.

Fluorescent lights do not fare much better. While brighter, as much as 50% of the energy they use is also wasted. Like empty calories, they might provide the immediate fuel you need, but aren’t the most nutritious option. What separates the old from the new is a matter of efficiency. LED lighting can be up to 95% efficient. The energy is emitted as the light we need, rather than the heat we don’t (or at least can’t make use of.)

Led Lighting uses lumens as opposed to watts. A single lumen equates to the light emitted from one candle. We understand brightness in terms of how far we are from its source.  Lux (lx) is a measurement of brightness that takes into account the proximity to the light source, as well as its range of distribution. One lux is the equivalent of one lumen per m2 (Lm/m2)

There are 4 components to Brightness:

  1. Lumen Output: the flow of light coming out (e.g. 700 lm for a 10W LED, warm white LED will emit lower lm than natural white colour)
  2. Distribution: How that light is spread over an area(cone, beam angle)
  3. Distance: at eye level, from each light source.
  4. Cumulative light: The total amount of light from all light sources in the area, (number of lights, light output, and how much reflected light and how much is being absorbed.)

If the distribution of light was square, the beam angle was 45 degrees, and if you doubled the distance, the light intensity (lux) would be a quarter. The light now illuminates 4 times as much space. If it is 3x the distance, then lux will only be 1/9th of the brightness at 1m. Beam angle is also important. A narrow (60o) beam will create pools of light in a low ceiling room, but would be ok in a higher ceiling environment.

A lighting diagram helps illustrate this. The image shows the lx at varying distance from the source. This light provides 1385lx at 0.945m from the luminaire, and only 351.3lx at 2.83m and if this is in a room that means you will have enough light for reading, typing or enquiry desks, but not enough for medium inspection work. The light distribution can be shown in polar charts, solid polar charts or in isocharts.

DistributionLight

Brightness will differ depending on the pattern of distribution, given that the area will be larger for a particular lumen output. As you can see, as the illuminated area is further from the light, there are less lumens (i.e. less bright). With multiple lights, it becomes more complicated.

What this means is that a room with twice as many lights will not necessarily be twice as bright. How the light is distributed plays a significant role in how bright the room will be.

A cone like distributionConeDistribution Bat wing distributionBatWingDistribution A bat wing distribution as a solid modelBatWingDistribution

 

Can you convert watts of luminaire to lux?

Yes. It’s mathematically possible, but it requires knowing the luminous efficiency of the luminaire.

  • Incandescent bulbs are approximately 13% to 18%
  •  CFLs are roughly 60 lm/W.
  • Digital LED lighting provides over 100 lm/W.

Assessing the lx in any given room, without any light adsorbed by the room, involves a certain kind of equation. The illuminance Ev in lux (lx) is equal to the power P in watts (W)times the luminous efficacy η in lumens per watt (lm/W) divided by the surface area A in square meters (m2):

Ev(lx) = P(W) × η(lm/W) / A(m2)  This example shown is with 48 fluorescent T8 tubes in a 112m2 classroom, compared with  24 LED Downlights.

Lamps in room Type Watts per lamp Total Watts Floor Area Total Watts/m2 Luminous efficacy Lux (W x (lum/W)/m2)
2 x 24 36W-T8 43 2064 112.5 18.3 45 825
24 33W LED 33 792 112.5 7.07 100 704

What the table demonstrates is that LED lighting can almost cut in half the amount of energy required to effectively illuminate a room of classroom size.

Comparing Luminous Efficacy and Efficiency

Lighting fixture efficiency be calculated. It’s a ratio of the total amount light going out of the fixture to the total amount produced by the fixture in the first place.  So if a 100W incandescent light bulb produces 1,000 lumens, and the fixture gives out 700 lumens, the fixture’s efficiency rating would be 70%  300 out of the 1000 lumens would be wasted, counting for the 30% loss in efficiency.

Efficacy measures the amount of light produced per unit of electricity used to produce it. It is a term normally used in cases where the input and output units are different.

Light type Typical luminous efficacy  (lumens/watt)
Tungsten incandescent light bulb

12.5-17.5 lm/W

Halogen lamp

16-24 lm/W

Fluorescent lamp

45-75 lm/W

LED lamp

30-105 lm/W

Metal halide lamp

75-100 lm/W

High pressure sodium vapor lamp

85-150 lm/W

Low pressure sodium vapor lamp

100-200 lm/W

Mercury vapor lamp

35-65 lm/W

As you can see, it will take an incandescent light bulb 5 times as much electricity to pruduce the same lumen output as an LED lamp. Energy saving lamps have high luminous efficacy (more lumens per watt)

Brightness Standards

The Australian and New Zealand standard for brightness (AS/NZS 1680.2.2) suggests the following light levels (lux) required for various tasks.

  • 240 lx: Food preparation or counters for transactions.
  • 320 lx: Routine office tasks such as reading, typing or enquiry desks.
  • 400 lx: Medium level inspection such as fine woodwork or car assembly.
  • 600 lx: Proofreading or fine machine work.
Task / Room Type Recommended Illuminance (lx)
General tasks: typing, reading, writing

background

 320

160

Screen-based task-     keyboards

–     reference material

  • good, simple
  • average detail
  • poor, fine detail

–     background

–     microform reading

 160

240

320

600

160

20 – 40

Drawing offices-     drawing board

–     reference material

  • good, simple
  • poor, fine detail

–     background

600

320

600

240

Meeting roomsTraining/seminar rooms

Conference rooms/board rooms

320

240

240

Reception areas-     enquiry desk

–     entrance hall, lobby, foyer

320

160

Photocopying and printing room-     Intermittent

–     Sustained

–     Colour

160

240

240

Filing area-     clear detail

–     fine detail

240

320

LED lighting suffers a common complaint: most installations have been a replacement of the light globe in an existing fixture. The light globe has a “driver” which reduces the voltage from 240V to the voltage of the LED. Generally these are only 5 to 6W, and the lux is usually too low to read or do fine work.

To combat this, Digilight recommends a minimum of 10W for any downlight to provide at least 300lx at bench height if lights are spaced about 1.5m to 2m apart in 2.8m height room. Anything less than this and the homeowner or worker will need additional lighting, or multiple lights.

You will need a modern dimmer switch as the older style don’t generally support the lower amps that LED lights draw. The homeowner can dim the lights for a subdued ambience, but can turn the lights up full for activities such as reading.

Glare

Glare is the strain to our eyes caused by an excessively bright light. It causes discomfort glare (fatigue) and lengthy period can cause a thing known as disability glare, the temporary visual impairment. Windows are a common source of glare, more so than powered lighting. Glare from lighting is more likely to affect those who work with screen-based equipment- computers, televisions, etc. These tasks require closer proximity to the source of lighting,  so glare from it will be more likely to make an impact.

The standard (AS1680.1) and a Unified Glare Rating (UGR, which is also specified in ISO 8995:2002 Lighting of Indoor Workplaces) states that glare cannot be more than 19 for general offices. Glare can be more substantial in larger spaces where visual jobs are demanding and need continuous attention or the job is at or above horizontal, and where desk and countertops are not sufficiently lit.