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rienquepourlesyeux@free.fr

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Last update : 2015/09/21

Flashmeter and Lightmeter for smartphone and tablet



I offer you make for yourself a flash meter / lightmeter very accurate
for smartphone and tablet, for less than 60 Euros cost (around 80 Dollars).
Smartphones and tablets are unable to measure a lightning of flash
(light too brief event). They are not able to make measuring of the incident light,
and the reflected light measuring accuracy is about 1/3 EV at best.
Therefore it is mandatory to achieve a plug for measuring light correctly and precisely.

CAUTION ! I do not manufacture the module, it is yours to make it,
it is not worth making a request by mail.
If you have any questions, you should ask in the forum, I do not answer
at questions by email. Thank you for your understanding.

To start, here are the specifications of this module.

General specifications.

- Accuracy of 0.1 EV.
- Measurement in incident light, displayed in LUX and Foot candles, or
- Measurement in reflected light ( spot metering ) displayed in cd / m² and Foot lamberts.
- Display of speed, aperture and ISO by half EV or third EV.
- Aperture of F1 to F90.
- Speed of 1/8000s to 60s.
- Sensitivity ISO of 25 to 819200.
- Possiblity to add a ND filter in the calculation.
- Possiblity to add exposure compensation.
- Calculate the Depth of Field.
- Bluetooth Communication, for distances of a few meters between
the cell and, Smartphone or tablet. This allows for easy reading
the display of the Smartphone in the sun, or to make measurements in
places difficult to access.
- Automatic connection Bluetooth ( pairing code first : 1234)
- Enter easily in the pocket (handy for take it everywhere ) or around the neck.
- Powered by two AAA batteries ( autonomy 4 to 10 hours depending on usage ).
- Compatible with all Smartphones and Tablets with a Bluetooth connection.
- Possibility to work with Android and computer, but also iPhone, iPad and Windows Phone.
I only develop for Android and Windows computer, but I provided in the ZIP file,
in the download section of the website ( FlashmeterXXXX.zip ), a PDF description of
the Bluetooth commands to create yourself your own application.
- battery level of the module Displayed in the application.
- Can be fixed on smartphone with a jack for easy use.
- Updating of the Firmware of this module is possible ( improvement , new features).
- Android application is multilanguage, interface screen in English,
French and Spanish but other language is possible.
- Creating lighting scheme and diagram (LSP = Lightning Studio Photo)
in the Android Application.
This feature is very useful to save the settings of a photo shoot.
- Calculate the depth of field in the Android Application.

Specifications of the light meter.

- Range of incident measurement (ISO 100): 0.5 to 1.9 million of LUX, -3 to 19.5 EV.
- Range of reflected measurement (ISO 100): 1 to 1000000 Cd/m², 3 to 20 EV.
- Hold the values.
- Choice of light source for high accuracy.

Specifications of the flash meter.

- Range of incident measurement (ISO 100): 100-2300000 LUX 5 to 20 EV.
- Range of reflected measurement (ISO 100): 500-10000000 cd / m², 12 to 25 EV.
- Flash sync with audio jack.
- Cell for automatic detection of lightning flash.
- Flash can also triggered from the Smartphone.
- 3 ranges of possible measures: Low EV, Normal and High EV for accurate measurement.
If the measurement is outside the selected range, an indication "OVER EX" or "UNDER EX"
appears instead of the EV value. It is necessary to change the measurement range, in this case.
- Cumulative lightning flash (can cumulate 255 lightning flash).
- Choice of flash sync speed of 1/60s to 1/320s.

NB: in flash meter mode, measurements in LUX are LUX equivalents (same for all measures).

To begin the presentation of this small module, Photo01 is the sketch
on which I worked for four months to design this project.
Top right of each photo, I have placed the histogram, so that you can you
get an idea of ??the quality of measurements.
On Photo01, the histogram is relatively homogeneous, we see a still pic
in the shadows to the left and a peak in the highlights to the right.
The red arrow on Photo01 shows you why there is a peak in the shadows,
this is because the phone is black. the peak in the highlights (blue arrow)
is due to the reflection of the lamp on the table.
It is not possible to do better at exposure for this photo without risking
to losing information in the high or low lights.
It is the limit of the dynamic of my DSLR sensor (peharps, Nikon D4 can be
better).

I do not provide the schematic diagram, or the program source code Arduino to protect
the copyrigth of my module. I'm all for sharing but not for looting.
I propose to send you a printed circuit board and the Arduino code already compiled.
For more information, please read the article "Build the flashmeter" on the site.
Photo02, the module next to a Samsung Galaxy ACE2, the conection is Bluetooth,
there is no son between the module and the Smartphone.
Photo03, the module fixed on a 8 inches tablet through the audio jack .
On Photo02 and photo03, the histogram is to the right, this is normal, most
part of the image is white so it has a peak at approximately 1 - 1.5 EV relative to the center.
Both pictures were taken with a single studio flash behind a white umbrella.
Both pictures are properly exposed, no loss in shadows and highlights.

Photo04, it is a view of the android application on two types of smartphone.
In 1 and 2, the application on a Samsung Galaxy ACE 2 (Android 2.3).
On view 1 of the photo04, the application is in light meter mode.
On view 2 of the photo04, the application is flash meter mode.
As the screen of the Samsung Galaxy ACE 2 is small (3.8 inches), it suffices
to scroll up and down up on the screen, to access all settings.
On view 3 of the photo04, the application on a Galaxy Note 8 (Android 4.1.2).
The 8-inch screen of the Galaxy Note 8 allows to see all parameters at same time.

This Android app is available in the Google Play Store:
search "Flashmeter and Lightmeter" or clic HERE

Photo05, this is the description of all the options and menus of the application for Android.
- 1: What is the status. during the search and connection to the Bluetooth module,
informations are yellow. Once connected, the informations are green.
The battery level of the module is sent every 30 seconds.
- 2: In incident measurement mode,we have LUX, Foot Candles,
the value of EV and finally in yellow,
this is the difference between the measured EV value and the EV value calculated by
the settings of the speed, aperture and sensitivity.
In reflected light Mode, we have Cd/m², Foot Lamberts, the value of EV and still
in yellow, the difference in EV, between the measured value and the calculated value.
This value in yellow, gives a precious indication of overexposure or underexposure
between the measured value and the calculated values ??of speed, aperture and sensibility
that we will adjust on the DSLR.
This indication will value of -0.2 EV, -0.1 EV, 0 EV, +0.1 EV or +0.2 EV.
CAUTION ! We must reverse this value to know if we have slightly underexposed or overexposed the photo.
Why ? if the measured value is 6.2 EV for example, and the calculation of the speed,
aperture and sensitivity, gives 6.3 EV, the value will be yellow with a value of +0.1 EV.
But as the value adjustments of the DSLR is given to 6.3 EV, but the real value is 6.2 EV, then
it will underexpose our photo of 0.1 EV. That's why we invert the sign.
- 3: We choose between the light meter mode, the flash meter mode, DoF (calculate Depth Of Field) and
LSP (Lighting Studio Photo for create shooting light diagram). If we want reduce the shadows with
a flash, and natural light (fill-in). We can make our measurement in light meter mode, for proper adjustment
speed and ISO.
In Flashmeter mode, Adjust your flash, to match the same aperture that you have measured in
light meter mode and thus reduce shadows.
- 4 : This is the speed setting (when the auto speed is not activated 8), by clicking on it
This brings up a list, that lets you select a speed between 1/8000s and 60s in steps of 1/3 EV
or 1/2 EV (according to setting 17).
- 5: This is the setting of the aperture ( when the auto aperture is not enabled 8) , by clicking on it
This brings up a list from which to choose an aperture between F1 and F90 per 1 /3 EV
or 1/2 EV (according to setting 18) .
- 6 : This is the sensitivity ISO setting (when the auto sensitivity is not enabled 8) , by clicking on it
This brings up a list from which to choose an ISO sensitivity between ISO 25 and 112400 per 1 /3 EV
or 1/2 EV (according to setting 19) .
- 7: Choosing a ND filter (Neutral Density Filter). by clicking on it, a list of different values ??of
ND filter appears. you can choose a value between ND2 and ND8192 (or none).
- 8 This is the parameter that permit to choose the measurement who vary, between the speed,
the aperture or the ISO sensitivity.
NB: In Flash meter mode, you cannot choose the speed setting as variant , simply because, with a
flash, this is the aperture and the ISO sensitivity who determines the exposure .
By cons you need that the speed is set to the flash sync.
- 9: This parameter is only displayed in light meter mode.
It allows you to choose the type of continuous light to measuring .
Commercial light meter do not have this setting, simply because, a correction filter
is placed before the photodiode. This type of correction filter is a simple piece of plastic,
square or round, specially processed to correct the response curve of the photodiode to
the different wavelengths of the light.
This filter costing a couple of dollars if you buy in quantity by 1000 . But if we buy the unit,
its price is several tens of Euros, the price with shipping is around 40 to 50 Euros !
So, I made the choice not to use it, but to have good accuracy, he choose the type
of light There are 8 types of light:
Sun (clear sky, the light is white / yellow).
Cloudy (overcast sky, no sun, the light is white/blue).
Shade (the natural light is the source of light, but the subject is in the shade).
Tungsten (incandescent bulb, same as halogen bulbs).
Warm fluorescent (neon tube or fluorescent lamp energy saving, of 2700°K to 3500°K, white/yellow light).
Cool neon (fluoresecente energy saving lamp, of 3500°K to 6500°K, white/blue light).
LED warm (LED lamp 2700°K to 3500°K, white/yellow light).
Cool LED (LED lamp 3500°K to 6500°K, white/blue light).
If you have trouble understanding why the photodiodes do not have identical responses following
lights sources, look Photo06.
- 10: This setting appears in flash meter mode and allows you to choose the measurement sensitivity of
the lightning flash.
The analog / digital converter in Arduino has only a 10-bit converter.
To get a good measurement, I put 3 choices of sensibility for measurement: LOW EV, Normal and HIGH EV.
It was not possible, same as in light meter, to change the sensibility mode automaticaly,
because of a lightning flash has a too short a time. If you choose a wrong choice during the measurement,
instead of EV, will be displayed UNDER EX (therefore choose for example: LOW EV) or
OVER EX (therefore choose for example HIGH EV).
- 11: by clicking on this button, all the lightning flash are cumulative.
It can accumulate up to 255 lightning flash.
To stop accumulating lightning flash, press this button again.
- 12: In light meter mode, this button allows you to store values
??temporarily displayed until you press it again.
- 13: Start the flash, this button can trigger all flash, if you use the flash sync socket.
- 14: By clicking on this button, you pass in reflected light, by clicking the button again,
you returns in incident light. In reflected light, the diffuser must be removed and
replaced by the black hooded of audio jack, for make spot meter measurements (angle of 23 degrees),
without this hooded, the measure will not be good, because of the light ambient.
- 15: Choose whether to use the diffusor. This button is only available in incident light measurement.
Make measurements without the diffuser allows a better accuracy in the orientation of the sensor
relative to the light source.
- 16: You can choose to use the EV compensation. This parameter is useful if you want to voluntarily
underexpose or overexpose your photos, to shoot in Low Key or High Key for example. By clicking on the
exposure compensation, a menu appears and allows you to select a value from -2 EV to +2 EV in 0.1 EV.
- 17: If you set the speed on your DSLR in increments of 1/2 EV or 1/3 EV .
- 18: If you set the aperture on your camera in increments of 1/2 EV or 1/3 EV .
- 19: If you set the sensitivity on your reflex in increments of 1/2 EV or 1/3 EV .
- 20: When the Android application starts on phone or tablet, Bluetooth is automatically activated.
By against, if you wish, when you quit the application, you can leave active the Bluetooth
by deselecting this setting.
- 21: When the Android application is started,
the screen of the smartphone or tablet not longer goes into standby mode.
This avoids having to press the button to turn the screen permanently.
By cons, to save energy, you can choose to leave the smartphone to standby
automatically (default usage of your smartphone) by unchecking this option.

Photo06, The calculator of depth of field to the left. This option is useful
and avoids having to leave the application to start another application when you want
to have information on the depth of field.
On Photo06, at right, this is the LSP (Lighting Studio Photo) for creating lighting diagrams.
You can change the size, angle and position of the images and text. Each image has a
associated text that provides the possibility to record values (height, distance, settings, ...).
Functions: load, save, auto save.

We have finished making the rounds of all the options.
Now I 'll try to give you some explanations about the sources of light and the perception of
the light by a photodiode .

Photo07, at the top left, I shows the frequency response curve of the photodiode BPW21R.
it is the same as all others photodiodes, it has a bell shape. Photodiodes can measure
light frequencies that our eyes cannot perceive.
As can be seen, with a fluorescent lamp for example, it have spectral response with peaks.
This translates to our eye, like a good light output (because the tip is in the spectrum
visible to the human eye) but in reality for the photodiode, a fluorescent bulb will always appear
less powerful than tungsten bulb or the sun.
The only way is to use a Correction filter and measure only the visible spectrum to the human eye.
In Red , I show you an ideal filter would allow a measurement of the light corresponding to
the perception our eyes.

Photo test of the flash meter.

Photo08, a first test of the flash meter: Transparent glasses on a white Plexiglas (white/gray).
One studio flash directed on a white ceiling as a good reflector.
Top right histogram, the curve is almost completely right
(normal, 90% of the image is very clear and white).
By cons, there is no peak at the far right, it means that we have lost no detail in high
lights. We can therefore work in post-production this image, without any loss of information.

Photo09, a few Christmas gifts for children.
The scene is lit by two studio flashes (left and right) and both directed to a ceiling
white to serve as a diffuser.
This allows for a homogeneous, encompassing and soft enough natural light,.
This type of lighting scheme is not bad for opening gifts by the children, this allow to
photograph childrens in all the room and in all positions.
To return to Photo09, she has nothing exceptional, it has a very uniform histogram. there is some detail in
the shadows (black underside of the car), some details in highlights (whites are white
not saturated). Note: the small peak to the right in the histogram in the highlights.
This peak is due to the reflection of the flash on the hood of the car (red arrow).

Photo10, the Yule log.
The histogram is homogeneous. No loss in the highlights, even in bright reflections
on the yule log. On the back wall, you see even a shadow line on the white. This proves that one has no
lost information about this picture.

Test of the light meter in incident light.

The big advantage of the measurement in incident light
(measurement towards the DSLR and/or towards the light source)
compared to our DSLR who measure in reflected light (towards the subject), is that the color and
reflection of the subject are not taken into account.
To show that our DSLR in measurement of reflected light does not always give a good exposure, but
also to show you that this small Flashmeter/lightmeter Module works very well,
here's a little series of photos.
For this little photo shoot, a dark wood statuette reflective, a beautiful married in white dress
made with printer paper (I know I have a lot of talent !), a dark background (sliding dark brown doors),
and also a white paper.
The subjects are placed between two room of open house (american kitchen).
For lighting, Photo11, Photo12, Photo13, Photo14, the roller shutter between the subjects and the background was
down to make the dark background and have the subjects in the shade (roller shutter at left).
The light source comes from the kitchen through the window to the left of DSLR. No sun , the sky is cloudy.
- First photo, Photo11 , made with DSLR in Aperture Priority mode.
The background is dark and the subject too, the DSLR has overexposed the photo.
The histogram remains correct but does not reflect any light atmosphere.
Note: the peak right in the high lights, it is the white paper under the statuette.
The white paper has lost information in the whites.
- Second photo, Photo12 made with the light meter in incident light directed to the DSLR.
The histogram is completely to the left, this is normal, because the background and the subject is dark colors.
This photo is very faithful to the ambient light (subject and background are in shadow).
- Photo13, the famous statuette and my beautiful married (she is beautiful !).
DSLR in aperture priority mode.
The histogram shows a tip at the right. We have a very considerable loss of information in the whites.
The photo is not good at all. Even in RAW format, it will not be possible to correct
loss of information in the whites.
- Photo14 , the same scene but measured with the light meter still in incident light measurement.
The histogram is perfect. No loss of information in the lower lights and high lights.
- Photo15 , the bride on a white background. The DSLR in Aperture priority.
The image is normally white ( background and subject),
the DSLR underexpose the image, for match the center of the histogram as possible.
Suddenly the white turned gray . This is the well-known pictures in the mountains when there is snow.
This is not good at all , we lost colorimetry. it is possible to correct this photo in RAW format,
but not in JPG format, the correction will necessarily affect the color rendering.
- Photo16, measure with the light meter, whites are true white. The exposure is correct.
You can see the usefulness of a lightmeter for a wedding.
- Photo17, this is my beautiful married on a dark background. The DSLR is in aperture priority.
The ambiant light is not natural, this is a halogen light spot .
We see very clearly in the histogram that we lost information in the high lights.
- Photo18, measure with the light meter. the light is halogen type. On the light meter I set the type parameter
light on TUNGSTEN, instead of SHADE. The histogram is well distributed over the entire dynamic range.
Halogen lighting have a lightly yellow light, it is present on white.
- Photo19, here is a very bad picture. You can easily replace the bird with a true bride.
Imagine a photo shoot with a bride, the sky is nice with plenty of sunshine . You have chosen a place
with trees, flowers. This is what could happen to you, if your DSLR is in automatic or semi-automatic mode.
Some say:
it does not matter, I do a test photo, I look at my histogram, and I correct it, so no need
of light meter.
This is true in this case, but if your subject is a beautiful Harley with beautiful chromes and sun,
it is obvious that you will have a peak to the right of the histogram (sun glare), but how to be sure
to have the best peak at right of the histogram ?
Others say:
I know this problems of differences between background and subject, and in some cases I apply +1 EV or -1 EV.
This will give results almost good but not correct.
The lightmeter was invented with the film camera, with the digital age, one might think that we no longer needed.
Yet, in many situations, it still has its uses.

Test of the light meter in reflected light .

I will not show you a lot of test in reflected light because there is no difference between the measurement
made by the DSLR and the lightmeter. To use the meter in reflected light, remove the diffuser
white and put the black hooded (and also change the metering mode in the Android application).
The black hooded has an angle measuring 23 degrees . If the light meter is placed in the same
location as the DSLR towards the subject, this corresponds to a measurement at 105mm with a full frame
and at 70mm with an APS- C .
If you want to measure smaller areas on the subject or scene, you will approach to the area to
measure (without creating spurious shadows). You will allow spot light measures of this area .
- Photo20 , DSLR in aperture priority, whites are gray, the colorimetry is not good , the picture is
underexposed .
- Photo21, measuring reflected light with the light meter, the picture is under exposed.
The light is natural and comes from a window facing the subject. Light varies slightly due to
the passage of clouds, that is why there was a very slight difference in illumination between the Photo20
and photo21, but it you can see that the setting provided by the light meter and the DSLR,
had given the same indications.
- Photo22, measurement made with the incident light meter for compare.
The histogram is much more to the right, this is normal, the background is light gray, the bride is white.
The photo is correctly exposed, colorimetry is respected.
- Photo23 and Photo24, two new pictures to show you the difference between the measurement made by our
DSLR and the one made by the lightmeter.
Photo23, white fluorescent light energy saving (about 5000-6000°K). Adjusting the Lightmeter on
Fluorescent cool directed towards the SLR with diffuser in incident light.
Photo24, winter sunshine outside. Adjusting the Lightmeter on sun and directed towards the SLR with diffuser
in incident light.

- Photo25, this is the app for Windows computer. GUI is not very stylish but
the function of this application is to be especially accurate and functional.
The font size is large, to allow reading values ??even, from several meters computer.
To measure light with this cell or this module, and a computer,
it is necessary that your computer is equipped with Bluetooth or a Bluetooth adapter.
The computer program for Windows is included in the ZIP file of the flashmeter.

This cell for measuring light is the first cell (and the only one) who is a real flash meter and meter,
for Smartphone, tablet and computer with Bluetooth.


I finished to present this flashmètre / lightmeter for smartphone and tablet.
This is not a competitor of my "Photographer Tools" because this lightmeter / flashmeter is not as much possibilities,
but it is small and easily hold in your pocket.
If you are interested by this project, read "Build the flashmeter" in the left menu of the site.



If you have any problems, if you have any questions, you can ask it in the forum.
I do not answer questions by email, only questions in the forum.
Sorry for my poor english, if you have correction, don't hesitate to post it in the forum,
after I will perform a correction.





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Reproduction in whole or in part, photographs and texts, without permission, is strictly prohibited.
Any commercial use of this arrangement is subject to intellectual property rights.
So thank you not to reproduce or distribute internet images and texts of this DIY.
Implementation and use of this DIY is allowed in a personal or school environment.




Copyright 2014 ThierryD - http://rienquepourlesyeux.free.fr
First update 08/01/2014
Last update 29/03/2014
Reproduction prohibited

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