Project tutorial
Light Meter To Measure Plant Growth Conditions

Light Meter To Measure Plant Growth Conditions © GPL3+

Light sensor to measure from 0 to 55,000 lux, which is a good range for measuring from the darkest room in a house to the brightest day.

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Components and supplies

11026 02
Jumper wires (generic)
Rg light a
RobotGeek Light Sensor
A000066 iso both
Arduino UNO & Genuino UNO
Grove-LCD RGB Backlight v4.0
Battery holder + wire to power the Arduino.
AA batteries

About this project


IMPORTANT CAVEAT - PLEASE READ: This work is carried out as a fun side-project by a scientist in a University competition on building electronics for biomedical research (me). However, I would like to mention at the outset, that I am a botanist, and not a medic.

In making this light meter to consider the effects of light on human health, I am just having fun, and investigating the subject to enlighten myself. It might be all wrong, so please take it with a fair sized pinch of salt.

I've written this up here in case anybody else would like to join the fun. Please think carefully if you are doing this, and play safe, especially with exposure to strong sunlight and very bright artificial lights. I am not qualified to give medical advice and the written information on this page does not constitute medical advice.

Having said that - here is the fun. I hope you enjoy it.

This project is an investigation into the effect of light intensity on the health of plants and people, or more specifically, gardens and gardeners.

I have spent my life moving back and forth between the north of the UK and the south, and even as far as France, and I have always been puzzled about the incredible effect that geographical location has on plant growth in my gardens, and on the physical health of myself and my friends.

As a child, I moved from Glasgow to Paris and back before the age of 9 and was acutely aware of the changes that I saw my own physiology and on the plant life around me. This same observation held every time I visited Scotland as an adult, through various seasons and changes of weather, and then reversed as I returned to work in the south. In recent years, as my community and I have matured, I have become aware that certain more serious health problems are more common in the north, and that these are almost certainly brought on partly by lack of sun exposure. At the same time, I have now become a garden owner myself. I now struggle daily with very different gardening challenges in the south, from those of my garden-owning friends in the north.

Throughout this process, I have noticed that gardeners and dog walkers seem disproportionately insulated from the health problems associated with northern living. This is presumably because their hobbies force them outside every day, regardless of the weather, though there are many possible reasons for the correlation.

I know that the difference in human health and plant life, between north and south, is almost certainly to do with light levels. However, I am constantly frustrated by my own inability to accurately measure light intensity with my own eyes. I feel that if I could properly judge how much sunlight I have had in each day, I could make sure to get enough, and teach others to do the same. I would then also be far better able to judge which spots would give a good home to which plant species in my garden. However, this is not an easy call to make.

The difficulty in judging light intensity is caused by the accommodation of the pupil of the eye. The pupils adjust naturally to light intensity. Therefore in any given situation, there seems to be just enough light. The very adaptability of the human being to living in any light intensity, makes it just a little harder for us to live really well, in any light intensity.

My hope in this project is to build a light meter that will enable me to accurately measure how much light there is in my environment in a range of different situations and at different times of day and year. My hope is that this will better enable me to manage my own physiology and also my own garden, with much less guesswork. I am living in Cambridge, but I am helped in the project by my life-long friend Catriona Ferris, who lives in the west of Scotland. She is going to take matching readings to investigate the light intensity in her own environment. Catriona and I are both keen gardeners and we have already investigated and compared most of the other growing conditions in our very different gardens, discussing every detail over Facebook. This year we will investigate light conditions, to try to find insight into how light intensity affects not just our plants, but also our own lives and those of our communities.

The design that I have chosen for the light-meter is built from an Arduino and associated parts. My hope is that if we find really interesting results, then this could be turned into a population-level educational tool. It could perhaps be distributed via a magazine like Gardeners' World Magazine or via a TV program like Springwatch, underpinning a national study of light levels in lives and gardens. That way the keen maker-teenagers of the country could build these machines and educate everybody else around them about the nature of light, and its effect on their gardens and health.


Visit the hardware summary page to find out about the construction of the light meter.

5 jumper leads, male to female

The light sensor measures from 0 to about 55,000 lux, which is a very good range for measuring from the darkest room in a house to the brightest day outside.

Here are some initial images of the hardware.


Visit the software summary page to find the code that runs on the light meter.

Installation, Maintenance and Testing Guide

Using the machine is very easy. Just plug it all together as shown in the hardware summary page. Plug into the computer with the Arduino's USB cable. Run the Arduino software and paste the code above into the code window. Then press the button (top left) to upload the code to the Arduino. Then unplug from the computer and plug in the battery and walk outside. The machine should automatically start displaying the light intensity on the LCD screen. The table below gives some example readings.

Maps of test areas

To see the results of our experiments please visit the following links:


I have no claim on this design. The parts are all commercially available, specifically to do this, and I am just using them for my own fun.


This project produced some very interesting information, particularly at the Cambridge part of the test. The results brought significant quality-of-life improvements for some of the participants.

Our biggest first conclusion in Cambridge, was that our shortage of sunlight in winter was mainly due to the cold weather, and not the lack of light outside.

In the winter time in Cambridge, there is strong sunlight outside most of the time and it is often dry and cold. Because of the very cold temperatures, we were just not going outside to enjoy the sunshine.

To improve things, we bought some quite hi-tech hiking clothes (fleece lined wind-proof over-trousers, that looked a bit like chinos.) This enabled us to maximise heat retention, and to start spending much more time out of doors. This brought significant benefits to general health, through increased sun exposure, exercise, and social time with neighbours. The benefits also extended to children and other family members who followed suit.

This improvement was in the Cambridge end of the experiment. It was not tested in Scotland, where heavy rain and high humidity makes spending time out of doors in winter much trickier.

At the Cambridge end of the experiment, we also were surprised to realise that eyestrain from computer screen use was partly down to the very low light levels indoors (about 30 lux all year round). This compared with a light level of around 700 lux in the University laboratory. This prompted us to change the lighting arrangements in home office rooms to improve eyestrain, which helped a lot.

One of the biggest surprises in both Cambridge and Fenwick, was about the extreme brightness of summer sunshine. Our light meter would not measure above about 55000 Lux, but for much of the summer the sunlight was right up at this level. This is a huge difference from the 30 lux that we were used to when sitting indoors. The realisation of this huge difference has helped us to balance our sun exposure to avoid burning in summer, and also to avoid the effects of lack of sun exposure in winter.

This experiment was very interesting indeed, and brought tangible benefits particularly to the participants in Cambridge.

Further results from the winter of 2018 - 2019

Further interesting results have been found from the continuation of this experiment in Cambridge during the winter of 2018 - 2019.

This winter I had great plans to get my warm outdoor gear on and enjoy the winter sunshine, which my light meter assured me was there outside to be enjoyed.

However, towards the end of November I again found the usual winter feeling of my body entering a quiet hibernation. I think that medics would call this malaise and loss of appetite, but all of the pet tortoises that I have known definitely thought of it as hibernation, and as far as I can see, it is the same thing.

This hibernation feeling is not what's wanted at Christmas, just as the world is full of musical concerts and school nativity plays to be enjoyed. I was frustrated to discover that it still arrived this year, even though I went out every day and sat in strong direct sunlight.

The light meter showed that that winter direct sunlight is above 55,000 lux, so I was certainly getting plenty of light, but the light was not doing its job.

The next experiment that I tried was to use a daylight spectrum lamp, which is specifically designed for seasonal affective disorder sufferers and produces 10,000 lux. The lamp was made by Lumie some years ago and takes a 70W/840/A/4P/LL daylight spectrum light bulb. The lamp produces 10,000 lux if I put the meter's detector right against the lamp shade, and the light intensity diminishes to about 2000 lux about 20cm away.

The light level of the lamp is significantly below the intensity of light that was available in the garden, but the different quality of the artificial light seemed to liven me up no end, and brought me out of hibernation again. I have only done this experiment once, so to get the necessary three repetitions, I will do it next year and the year after, to check.

However the thing that I seem to have worked out, is that for some people, like me, it is possible that winter sunshine, however bright, is not quite as good as the artificial daylight lamp kind for keeping a person lively in December. This is very useful to know.

It also means that, using this light meter, it is possible for a person to study their own responses to winter light and report these back to their GP. This can result in a formal diagnosis of seasonal affective disorder, which could be really useful in severe cases, and save a GP a whole lot of detective work, which has got to be a good thing.

Anyway, there we are for this year.

Thanks again to the OpenPlant Fund. They provided the electronics kit as part of the 2017 Biomaker competition, and it was that box of kit that spurred me on to try this experiment.

Now roll on spring...


Code for light meter. C/C++
This code enable the light meter to display light intensity in lux.
#include <Wire.h>
#include <BH1750.h>
#include "rgb_lcd.h"
rgb_lcd lcd;
const int colorR = 0;
const int colorG = 0;
const int colorB = 0;
BH1750 lightMeter;
void setup() {
// set up the LCD's number of columns and rows:
lcd.begin(16, 2);
lcd.setRGB(colorR, colorG, colorB);
// Print a message to the LCD.
lcd.print("Light intensity");
void loop() {
uint16_t lux = lightMeter.readLightLevel();
Serial.print("Light: ");
Serial.println(" lx");
// Clear the bottom line of the LCD
lcd.print("                  ");
// For LCD: set the cursor to column 0, line 1
// (note: line 1 is the second row, since counting begins with 0):
lcd.setCursor(0, 1);
// print the number of seconds since reset:


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