Can I use "Luxometer" as a lightmeter?

<p>Hi Everyone.<br>

Among many Light meters available on the internet I found lot of "Luxometers", devices that measure the light and give You reading in <em>Lux. </em>Something like this: <br>

<a href="http://allegro.pl/swiatlomierz-luksomierz-lx-1020b-0-100-000-lux-fv-i2040569118.html">http://allegro.pl/swiatlomierz-luksomierz-lx-1020b-0-100-000-lux-fv-i2040569118.html</a><br>

These are probably to measure light in offices, workplace, etc. Seems to be very accurate...<br>

<strong>Can I use one of the "Luxometers" as a light meter especially for Old 6x9 camera and transparencies? How to convert<em> LUX</em> readings to photography-related values? Any experience?</strong><br>

Thanks, <br>

<em>Maciek</em></p>

<p>Convert Lux to EV:<br>

http://www.sekonic.com/Support/EVLuxFootCandleConversionChart.aspx</p>

<p>Then use an EV table:<br>

<a href="http://en.wikipedia.org/wiki/Exposure_value">http://en.wikipedia.org/wiki/Exposure_value</a></p>

<p>However, if the sensor is flat, it won't be as useful as a hemispherical sensor for photography. Unless you mostly photograph flat things.</p>

<p>Hey Matthew, thanks for help!<br>

I'm doing a research and I figured out that to reach EV 16 I would need meter that can measure up to 200.000<em> Lux. </em>Am I correct? <br>

I'm thinking about this one, it have dome over the sensor<br>

<a href="http://allegro.pl/profesjonalny-swiatlomierz-luksomierz-luksometr-i2043125199.html">http://allegro.pl/profesjonalny-swiatlomierz-luksomierz-luksometr-i2043125199.html</a><br>

Thanks again!<br>

M.S.</p>

<p>Convert Lux to EV:<br /><a rel="nofollow" href="http://www.sekonic.com/Support/EVLuxFootCandleConversionChart.aspx" target="_blank">http://www.sekonic.com/Support/EVLuxFootCandleConversionChart.aspx</a></p>

<p>Whilst the method is correct, as a pedantic point of order, I must point out that it is LV (light value) not EV (exposure value). At ISO 100 the figures are the same, at other speeds, they are not.</p>

<p>The text to the left of the table does point this out but it is a common mistake to mix up the two terms.</p>

<p>Not pedantry but necessary fact.</p>

<p>OK, I now see that you are looking at these because they are cheap. A search on ebay reveals similar meters selling for less than US$20. I am not confident that anything that cheap could be accurate enough for transparency photography.</p>

<p>You may be better off finding a used photographic light meter. Or, as some people do, use an inexpensive digital camera to determine the exposure.</p>

<p>Low price does not have to mean reduced accuracy. Such a meter will require you to convert a raw reading into units you can use. For reasons of speed and convenience, a meter designed for photography is thus to be preferred. As it happens, I have an inexpensive one in the classified ads: http://www.photo.net/gc/view-one?classified_ad_id=1370230 . There are of course plenty of other options.</p>

<p>Thanks for the responses.<br>

I was thinking that it may be extremely accurate since it's counting in thousands... <br>

<strong>Do You know if anyone bother with these devices or I should save money for good lightmeter.</strong><br>

I'm not worry about transferring values, I'll print the chart, but need an accurate measurements...</p>

<p>Its counting in thousands need not mean anything. A hopelessly inaccurate instrument may give a read-out to seven decimal places. I shall advise you to search for reviews of particular models.</p>

<blockquote>

<p>I am not confident that anything that cheap could be accurate enough for transparency photography.</p>

</blockquote>

<p>They're perfectly accurate enough. It's just that anything with a "photographic" label put on it has its price inflated to about 3 times what it's actually worth. A lightmeter would need to have an error of over 25% to make any noticeable difference to a camera exposure.</p>

<p>However, some of those industrial luxmeters don't go high enough to be used in bright daylight. The highest daylight luxmeter reading I've taken is just under 150,000 lx, and some meters only go up to 100,000 lx or less. The above reading was taken at a latitude of 51 degrees, so if you're nearer to the equator you'll probably get even higher readings. 150,000 lx is roughly equivalent to an EV of 16 with 100 ISO. There's a handy <a href="http://www.sekonic.com/Support/EVLuxFootCandleConversionChart.aspx">conversion chart here at Sekonic's site.</a> Anyway, the point is, if you're going to use one of these luxmeters as an incident exposure meter, just make sure it has a high enough range for daylight.</p>

<p>Just out of interest, and surprisingly, the highest daylight readings I get are when there's a scattering of white cloud and not under open blue skies. The white clouds act to reflect sunlight and so add to the reading from direct sun. The very brightest readings I've got are from when the sun just clips the edge of a light cloud.</p>

<p>Doh! Just seen that my link has been given at least twice already.</p>

<p>You can translate lux into f and t and iso in this way:</p>

<p>E = 270 * f^2 / (asa * t)<br>

Where E is the iluminance in lux.<br>

f the number f<br>

asa is the asa part of the ISO photographic sensitivity (not the DIN).<br>

t is the exposure time. TIME not Speed. (1/60 not 60)</p>

<p>As rule of thumb: 1000 lux are f:2,8 for t 1/50 with ISO 100/21<br>

(This is the base for cinematography calculations).</p>

<p>BUT: if your lightmeter can measure luminances (what in old photographic books are called "reflected light", actually brillance) then the equation is this:</p>

<p>L = 15.41 * f^2 / (asa * t)<br>

Where L is the luminance in candles per sqare meter (Cd/m2)</p>

 

<p>I just ordered one from ebay that looks like the first one Maciek referenced. Only US$22 including shipping. My interpretation of the specs, it is only usable from about 6-15 EV(100). I will compare it with my Minolta.</p>

<blockquote>

<p>" Low price does not have to mean reduced accuracy. Such a meter will require you to convert a raw reading into units you can use. For reasons of speed and convenience, a meter designed for photography is thus to be preferred. As it happens, I have an inexpensive one in the classified ads: <a rel="nofollow" href="../gc/view-one?classified_ad_id=1370230">http://www.photo.net/gc/view-one?classified_ad_id=1370230</a> . There are of course plenty of other options. "<br>

I believe low price does mean low quality and reduced accuracy. The calculation circuit needed to convert reading to f/stop shutter speed is cheap if produced in large quantity. <br>

<br /></p>

</blockquote>

 

<p>Thanks for the responses!<br>

One more thing: <strong>Dome shaped sensor will probably read lot of sky, do I have to modify it somehow?</strong> Maybe paint the top part of the dome? </p>

 

 

<blockquote>

<p>One more thing: <strong>Dome shaped sensor will probably read lot of sky, do I have to modify it somehow? </strong>Maybe paint the top part of the dome?</p>

</blockquote>

<p>No, your subject will be lit by a lot of sky also. </p>

<p><strong>When using an incident meter, always point the meter at the camera.</strong> This may be contrary to what the instructions will say since the Lux meters you referenced are mostly designed to measure light in work settings. There you normally place the meter flat since office workers paperwork is usually lays flat.</p>

<p>for most situation you want the dome receptor. the dome receptor and flat one give different reading about 4/10 of a stop difference.</p>

<p>According to the indication of Paco Rosso I understand that the relationship between the illuminance of a scene and its corresponding luminance is always equal to the ratio of 270 to 15.41 ?<br />That would be a fixed reflection factor for the scene.<br />I guess it can be really valid if one uses an 18% gray card or anything like that but not in the case of a real scene .<br />I try to learn about it and really the Paco's answer is the most precise and specific that I found on the subject. Thank you very much for sharing.</p>

<p>The relationship between illuminance and luminance is:<br /><br />L = (r*E)/PI</p>

<p>Where L is the luminance, in cd/m2. (Candles per square meter).<br>

E is illuminance in lux.<br />r is the reflection coefficient (The ratio between the luminance of a sample of the material divided by the luminance of the reference).<br />PI is PI, it is, 3.141599...</p>

<p> </p>

<p>First of all, thanks for the quick response. Fast and very accurate.<br>

I carefully follow your articles in Spanish (my native language) in your photography blog and many others written by you at different sites or as courses that can be read or downloaded from the Internet.<br>

I have some rather subtle doubts. I read in English without problem but write them, that's something different if I really want to convey the exact idea. (much more if it has included a dubt more than a idea !)<br>

I do not know if this site is permitted up a comment in a language other than English.(¿?)<br>

I study a little about your answer because not sure why intervenes PI number.<br>

My interest is more linked to the electronic portion of photometry, but I admire photography as an artistic discipline and its documentary value.<br>

<br />And that is why I value your articles because I learn a little (as a novice ) of photography and when is necessary the mathematical physicist rigor, you perfectly explicit about these.<br>

That's not easy to find on the Internet and helps a lot in learning, especially when that they want to learn has some knowledge of physics and mathematics which it is the place where all languages happily converge.<br>

Those blogs o artistic notes that are too oriented to photography itself ignored those concepts or not to give it true importance.<br>

<br />I will now review the concepts that told me and surely return with some more specific question .<br />Again, thank you very much.</p>

<p>About the PI factor.<br /><br />When a light beam arrives at a surface can do two things: bounce or splash.<br>

The light can bounce as the balls of billiard game. We call this "specular reflection". The light beam comes, and the light beams goes.<br /><br />Now think in a stream of water from a hose pipe against a fence. The stream of water do not "bounce" but "splash. When the light splash we call it "scater" or "diffuse reflection".<br /><br />So, a beam of light (a ray) arrives and change its distribution to a sphere. The change in energy is from a ray to a sphere. This is what the 1/PI factor explain, the change in intensity from the ray of light arriving to the figure to the sphere of light departing from the figure.</p>

<p> </p>

<p>Excellent. The analogy could not be better.<br>

As I said earlier I have no trouble reading English but convey an accurate idea costs too much and I use google translator.<br />As we know, textual translation is not always appropriate to express some subtleties of language that are necessary for proper context so I thought the following to not be clear if I upload a post in Spanish.<br>

I will try to express myself in short sentences, in such a way that the automatic translator requires less range "interpretation" and also upload it in English, I will include the original text in Spanish language in which I can express myself more clearly.<br>

Understood that the difference between the specular rebound and diffusion of the light beam on an object produces the need for the introduction of a correction factor, in this PI case, to convert the effect of light energy carried in a light beam when it becomes a spherical distribution, the following question arises me why consideration is made on a sphere or rather on a spherical cap and not on a circle every time when the light beam "hits" on the plane (jet water against the wall), actually what occurs is a flat figure and not a volume.<br>

I guess the general expression comes from that source model must be formed by a cone of light expressed in SR which in turn affects the inside of a spherical cap space and relationships arise from combining spherical aggregates with quadratic areas and lengths which they are obviously linear.<br>

Turning to the luminance, be noted that the measurement is performed against a reference luminance value "lit my doubt." All a flash!<br>

This is absolutely essential and yet, despite having quite sought in this regard, it is never explicitly stated.<br>

Now I add a question: there is a "standard" reference luminance or is chosen for convenience?<br />Perhaps because I am familiar with the issues RF, I tend to assume that it is something like dBm, which as you surely know perfectly, it is potentiometric reference that simplifies the end all calculations in the world of Radio communications and sound.<br>

If such were the case, there is no problem of understanding the concept of a "negative luminance value."<br>

Again, the concept of measurement with respect to a reference value of luminance is the "golden key" to the question.<br>

Having mastered these fundamental concepts convey what the source of discussion and exchange of ideas among a group of electronics hobbyists and technology and why friends environment is linked to photography.<br>

Thank you very much for your time and congratulations for the strength and clarity of explanations.<br>

Down, in spanish</p>

<p>Excelente. La analogía no podría ser mejor.<br>

Como expresé anteriormente no tengo problemas para leer en inglés pero transmitir una idea exacta me cuesta demasiado y uso el traductor de google. <br />Como sabemos, la traducción textual no siempre es adecuada para expresar algunas sutilezas del idioma que son necesarias para lograr un contexto correcto así que pensé en lo siguiente al no tener claro si puedo subir un post en español.<br>

Trataré de expresarme en frases cortas, de tal suerte que el traductor automático requiera menos rango "de interpretación" y además de subirlo en inglés, incluiré el texto original en español idioma en el cual podré expresarme con mayor claridad.<br>

Comprendido que la diferencia entre el rebote especular y la difusión del haz de luz sobre un objeto produce la necesidad de la introducción de un factor de corrección, en este caso PI, para convertir el efecto de la energía luminosa transportada en un rayo de luz cuando se convierte en una distribución esférica, me surge la siguiente duda: por qué la consideración se realiza sobre una esfera o mejor dicho sobre un casquete esférico y no sobre un círculo toda vez que cuando el haz de luz "impacta" sobre el plano (chorro de agua contra la pared), en realidad lo que produce es una figura plana y no un volumen.<br>

Supongo que la expresión general deviene de que el modelo de origen debe estar formado por un cono de luz expresado en SR que a su vez incide sobre el interior de un casquete esférico hueco y las relaciones surgen de combinar magnitudes esféricas con áreas cuadráticas y con longitudes que obviamente son lineales.<br>

Volviendo al tema de la luminancia, hacerme notar que la medición se realiza contra un valor de luminancia de referencia "iluminó mi duda". Todo un flash !<br>

Ese hecho es ABSOLUTAMENTE FUNDAMENTAL y sin embargo, a pesar de haber buscado bastante al respecto, jamás está explicitado.<br>

Ahora agrego una duda: hay una referencia "standard" de luminancia o se elige por conveniencia ?<br>

Quizás debido a que estoy familiarizado con las cuestiones de RF, tiendo a suponer que es algo así como el dBm, que como seguramente usted conocerá a la perfección, se trata de una referencia potenciométrica que simplifica al extremo todos los cálculos en el mundo de las radiocomunicaciones y del sonido.<br>

Si tal fuese el caso, no hay ningún problema de entender el concepto de una "luminancia de valor negativo".<br>

Nuevamente, el concepto de medición con respecto a un valor de referencia de luminancia, es la "llave de oro" para la cuestión.<br>

Una vez asimilado estos conceptos fundamentales transmitiré cuál es el origen de la discusión e intercambio de ideas entre un grupo de amigos aficionados a la electrónica y la tecnología y por qué está ligado al ambiente de la fotografía.<br>

Muchas gracias por su tiempo y las felicitaciones por la contundencia y claridad de las explicaciones.</p>