We’ve written a couple of recent articles about solar powered outside lights and portable LED lights suitable for traveling with. But the as yet unanswered question is ‘how much light is enough’? Please now continue reading.
If you find yourself in a hotel room that is too hot or cold, or too dim, it really helps if you can quote facts to the front desk in support of your complaint. As one who is sadly a battle-scarred veteran when it comes to complaining about air conditioning, I know the only way to respond to the ‘delaying tactics’ and platitudes of the front desk staff is to say ‘the air temperature out of the a/c vent is 75 degrees as measured by my temperature probe, so there’s no way it will cool my room down to 72 degrees’. Or to simply say ‘the temperature in my room was 78 degrees when I checked in – I measured this with my temperature probe, I’ve had the a/c on full for an hour, and now the temperature has actually increased to 80 degrees’ (or whatever other numbers apply).
But when it comes to saying ‘my room is too dim’ how do you back that claim up with facts? How do you measure the brightness/darkness in your room – its lightness?
Units of Light
There are two ways to measure light. The first measure is usually referred to as lumens, and it is a measure of the total light, for example, the total light emitted by a light bulb. Clearly, the more lumens that a light source gives out, the better. But the lumens given out by a light don’t directly relate to the actual brightness that you see when trying to read a page in a book. If you have a light right by you, shining on the book, even a weak light will help brighten up a small area, and similarly, if you are in a major meeting hall, a small light in the center of the ceiling will barely soften the shadows in the corners of the room.
This brings us to the measure of the intensity of the light, at a particular place – ie, at the place where you need the light. The intensity of the light in an area is measured in lux, and in case you are wondering, one lux is the light that results when one lumen of light is spread evenly over one square meter. The unit ‘lux’ is a metric unit, a more familiar US measure might be in foot candles – one foot candle measures the intensity of light when one lumen of light evenly illuminates one square foot. So to convert lux to foot candles, divide by 10.8; to convert foot candles to lux, multiple by 10.8 (eg 20 foot candles is the same as just over 200 lux).
As an analogy, lumens of light are like ounces of peanut butter in the jar. Lux of light are like the thickness of peanut butter when you spread it on bread. One ‘lumen’ of peanut butter might make one generously thick spread for one sandwich, or a disappointingly thin spread for three or four sandwiches.
So, when we want to measure the practical amount of light in the areas we need it, we talk about the lux intensity.
How Many Lux are Enough?
So, how many lux are considered normal or necessary? That depends on what you are wanting to do in the lit area. If you want to sleep, you want almost no lux. If you want to have a soft candle-lit romantic dinner, only a few. If you’re wanting to walk down the hallway without bumping into things and being able to clearly see the light switch and door handle when you approach the other end, 50 – 100 lux would be fine.
A normal living room, when you’re talking with friends or watching television, might be in the order of 100 – 200 lux, a dining room where you want to clearly see your food would be 300 – 400 lux, a kitchen where you want to see the food you’re cooking and not fumble when picking up knives and forks would be 750 lux and higher.
Clearly, ‘pitch dark’ is zero lux. At the other end of the scale, if you’re outdoors on a bright sunny day, and reaching for your sun-glasses, the chances are there is over 10,000 lux of light intensity about you.
If you’re wanting good light for detail work and reading a book, then you should have in the region of 500 – 1000 lux of light. The GSA standard for normal work station space is 500 lux. This Pacific Northwest National Laboratory paper distinguishes between light levels for computer type tasks, which it says can be satisfactorily performed in a 300 – 500 lux environment, and paper based reading tasks for which the norm is generally 750 – 1000 lux. OSHA specifies a minimum safe level of 310 lux (30 foot candles) in offices.
One more thing to keep in mind. Our eyes see light ‘logarithmically’ rather than linearly, and with our pupils changing the amount of light admitted, we only are particularly sensitive to light when either our pupils are at their smallest size and still the light keeps getting brighter beyond the level our pupils can compensate for, or if our pupils are at their largest size and still the light keeps getting darker. That also means each extra 100 lux doesn’t result in a similar difference in perceived brightness. So the difference between, for example, 200 and 300 lux isn’t as big as it seems on paper.
How to Measure Lux?
There’s as much art as science when it comes to measuring lux (and a related property, luminance), because there are big differences between average ‘area’ values and specific patches of bright light. So it isn’t just a case of how you measure and what you use to measure with, but also what you measure.
Sure, you can go out and buy a professional grade lux meter – Amazon sell them from $16 – $2000. They are easy to use – just put the light sensor where you want to measure the light intensity and read the value off the digital scale.
But, why buy another piece of ‘gear’ to add to your already full ‘road warrior kit’. The good news? There’s an app for that – iOS and Android smartphones can use the light sensitivity of their built in cameras to do double duty as a moderately . But be careful not to get a ‘light meter’ app that is designed to set exposure values for cameras rather than to show lux levels. EV readings are logarithmic and also depend on the speed of the film in the camera, lux levels are linear, so the conversion between them is complicated rather than simple, although if that is all you have, it might be helpful to know that, assuming an ISO/ASA rating of 100, then 300 lux are about 6.5EV, 600 lux are 7.5 and 1000 lux are 8.5 EV.
For our iPhone we tried Pocket Light Meter, but it only showed EV. The promising app named ‘Lux’ also only showed EVs. Third time is a charm, and ‘Lux Camera’ provided a lux meter in a free app. It also had credibly positive reviews.
So as to see what extra you get for your money if you actually pay money for an app, we bought a copy of what seemed to be a highly rated product, ‘Light Meter’, costing $1.99. This had indoor and outdoor settings, and no information about why there were two settings or what the change was to their measuring. The indoor setting was about half the value shown on the free ‘Lux Camera’ app, and the outdoor setting was almost twice the value (we did our tests indoors).
There was some consistency between the two EV apps and their measurements, and their two measurements, when converted to lux, tended to be a little higher than the Lux Camera app and significantly lower then the Light Meter app’s outdoor setting (and also way higher than the indoor value).
The difference in readings can partially be explained by different reading methodologies. Perhaps one program reads only a ‘spot’ value in the center of the frame while another reads an average light value across the entire frame. Bottom line – we saw no added value in paying $2 to buy an app, compared to the functionality of the free app.
With our Nexus 5 phone, we found an even broader range of free Android Lux meter apps in Google’s Play App store. We particularly liked the ‘Light Meter’ app and the ‘Lux Meter’ app, but both required their sensor to be calibrated against a reference value prior to giving accurate readings. We calibrated them against our iPhone, which is far from a ‘reference value’ quality reading to start with. There are so many free ones to choose from, so download several and see which you prefer.
One way to calibrate would be if you have an old camera or camera light meter that gives EV readings (or even using any modern camera and deriving the EV reading from the shutter speed, aperture and ISO settings), and then calibrating from that.
Whatever you use, you should then get a series of ‘real world’ readings around your office and in other spaces (and other hotels) so you not only have absolute lux values but also comparative ones when assessing the (in)adequacy of hotel lighting.
How to Use the Data to Complain
The good old-fashioned Human Eyeball Mark 1 does a perfectly adequate job of telling us if we’re in a place that is too dark or not. The problem comes when we try and externalize that to another person, who in turn says ‘no, it is perfectly bright enough, stop making a fuss’. We need some external quantitative validation. Hence the need for this measuring.
We would suggest that two important phrases to use in any complaint at a hotel are references to the General Services Administration and their 500 lux standard for office/task type lighting, and the OSHA minimum safe standard – a minimum of 310 lux. Whether valid or not, an implication that the hotel is violating a federal safety standard might get some attention and response.
Then refer to those and also the lighting levels in your office or other work environments, and maybe you’ve also started to collect some readings from better lit competing hotels as well that you can mention. ‘In my dining room at home, the light level is xxx lux, in the office it is xxx lux, and at the ABC hotel their room has a xxx lux lighting level. But here, your hotel room only has xxx lux – that is not only the dimmest of all these places, but way below both the GSA recommendation of 500 lux and the OSHA minimum safe standard of 310 lux.’
Of course, it could be argued that office lighting standards don’t apply to hotel rooms, but if you’re wanting to work or read in your room, then those are the applicable standards. You need a work or reading area with sufficient light. Surely that’s not too much to ask for, so go ahead and do so, with the ‘facts and figures’ at your fingertips to validate your request.