Are you pressing the "Zero" button? If you do then the signal might go negative.

Thanks for you reply Tuhin. I do press "zero". I am not supposed to press zero?

No. Please do not use that button.

When you press "Zero" you are essentially making a positive value to 0. i.e., you are making (lets say) 10 a.u. intensity to zero. So if your sample has less intensity (say 5) then the "zeroed" intensity will naturally go negative. In case you want to substract signal you can easily do it later manually. The subtraction procedure might not be straight forward due  inner filter effect.

I just measured it again without pressing zero and it is not showing any negative values anymore. You are 100% correct. Thank you

Dear Colleague,

If you really can measure negative fluorescence, then you should go to Stockholm to accept a new Nobel Price for the discovery of "Antiphotons", may be an analogon to ANTI-Materia.....Until you get the invitation from Stockholm follow your way by asking the question: WHY do I see a funny phenomenon? Many people can help you to find the bug. e.g.

Often you can make an error with the baseline or reference fluorescence. Even in full darkness any fluorimeter can indicate a positive or negative signal due to a non-precise setting of the instrument signal for complete darkness = zero light = no photons, but not necessarily NO CURRENT.

If the reference signal refers not to zero photons, but to a reference light beam, then you may be trapped by quenching-phenomena and you see less fluorescence.

An other trivial situation can be due to the setting of the zero light (or reference light) from where ever it can come. An other case is possible that your "lower signal" is not due to a different intensity of photons but due to any kind of electric offset due to a slightly wrong setting of the dark current in real darkness.

You will find the reason, which will be easily corrected. The most important thing is, that you DID OBSERVE THE PHENOMENON AS AN ARTEFACT and that you where taking actions to get rid of it. BRAVO. Good luck for your safe measurements leading to interesting scientific RESULTS. Any time you can contact me for funny questions about Bioenergetics, Photobiology and Spectroscopy:

[email protected]

Bioenergetics Laboratory

University of Geneva (Switzerland)

or at my home:

Hofgutweg 51B

CH 3400 BURGDORF

Switzerland

Negative and positive signals are always relative to an electronic ZERO-line. e.g. photomultipliers give primarily a negative photocurrent when they "see" light. Most instruments change the polarity a priory so that your measured light intensity is proportional to a positive current or to a positive voltage. Over 30 years I'm teaching Photobiology and Bioenergetics by telling the students: When ever you can measure a signal, then you are in business. All the rest is to find out if your measurement is an artefact or a real message from your sample. If it is a real signal you have to be interested in the SHAPE of that signal. Positive or negative, it doesn't matter. Even the amplitude doesn't matter. What matters is the SHAPE of the signal. If you hang a painting of PICASSO on the wall, but upside down (what can happen for Picassos pictures...) it will still and always be a picture of PICASSO.

The same with your signals. You are not interested if the voltage is going up or down, you are interested what your SIGNAL CHANGE means, not as a positive or negative voltage but as a meaning of a biological function. The SHAPE of the signal is important for you. The direction of the signal as positive or negative and shape as going up or down or oscillating is important. That's why it is important for novel publications to show the original experimental traces with the indication of the used instrument.

Eskimos are standing on their feet far up in the north, but Pinguins are not standing on their heads in the Antarktis. but both have their feet on the ground...... What you are measuring in science are SHAPES of signals the quantification depends on the UNITs and the polarity of the signal. Good luck. Reto J. Strasser. For questions:  [email protected] 

Dear Prof. Strasser, You must know that I am your fan!. Increadible, amazing and didactic answer. In addition, by the way, I totally agree with you, and the analogy with the Picasso picture was amazing. Best regards! 

Dear Colleague,

Thanks for your nice message. I did not know that I have somewhere in the world a FAN. May be that you do not know that I'm an old man (with a lot of fun in life) and not a fancy young lady. Tell me what your research is and what your questions are.

At the University of Geneva I printed about 20 volumes of my publications called "YELLOW BOOKS Strasser", similar to the other site "JIP-Test Strasser". All available for free. However they disappeared from the Internet even so they were not pornographie at all...If you are interested I could send you a CD with the pdfs of each volume of Yellow Books. Tell me when you are visiting Europa (for a meeting, congress etc), so may be we can see one each another. Meanwhile keep in touch. Questions of young people are always interesting and keep old scientists longer in action. By the way: I'm Swiss and I'm living in Switzerland. I'm as well Prof. extra-ordinaries in South Africa, where I'm going for one month every year. However I do not know how to change my address on researchgate......Science is easy, but administration NOT........

Best regards

Reto J. Strasser   [email protected]

Hello Prof Reto J Strasser

Though I have gone through a number of papers, still I want to learn OJIP and JIP test analysis from your publications. Would you please send me these books.

Dr Habib R Athar

Institute of Pure and Applied Biology

Bahauddin Zakariya University, Multan 60800, Pakistan 

e-mail [email protected] 

Dear Reto,

Great answer to researchers and students.

Prof. Baishnab C Tripathy

School of Life Sciences

Jawaharlal Nehru University

New Delhi 110067

Imdia

[email protected]