Srila Prabhupada often said that guru must be a TRANSPARENT media. Like prescription glasses: you can see Krsna through the transparent media of your prescription glasses (guru).

Such prescription glasses are extremely valuable, but if we insist that we must primarily see the glasses, we are just plain mistaken. Or if the glasses are not transparent, such glasses need to be rejected and a better pair picked up as soon as possible.

Sunglasses are no good, then?

I prefer rosy-tinted glasses, myself.

trans·par·ent (trns-pârnt, -pr-) KEY

ADJECTIVE:

1. Capable of transmitting light so that objects or images can be seen as if there were no intervening material. See Synonyms at clear.
2. Permeable to electromagnetic radiation of specified frequencies, as to visible light or radio waves.
3. So fine in texture that it can be seen through; sheer. See Synonyms at airy.
4. 1. Easily seen through or detected; obvious: transparent lies.
   2. Free from guile; candid or open: transparent sincerity.
5. Obsolete Shining through; luminous.

ERGO, when we see him we see Krsna. When we hear him we hear Krsna. When he's here, Krsna's here.

From the Science of Self-Realization (Chapter 8):

Therefore, as far as possible, one has to execute the order of one's spiritual master. That will enable one to progress. That is the essence of the favorable execution of KRSNa consciousness. In my old age, I have come to America, and I am trying to teach KRSNa consciousness, because my spiritual master gave me an order that I must do it. It is my duty. I do not know whether I shall be a success or failure. It doesn't matter; my duty is completed if I can present before you whatever I have heard from my spiritual master. This is called the favorable execution of KRSNa consciousness. Those who are actually serious should take the order of KRSNa through the representative of KRSNa as their entire life and soul. One who sticks to this principle is sure to progress. Caitanya MahAprabhu spoke in that way, and my spiritual master used to say, "The spiritual master is the transparent via medium." For example, I can see the letters of this book very nicely through these transparent eyeglasses, without which I cannot see because my eyes are defective. Similarly, our senses are all defective. We cannot see God with these eyes, we cannot hear Hare KRSNa with these ears, we cannot do anything without the via medium of the spiritual master. Just as a defective eye cannot see without the via medium of spectacles, so one cannot approach the Supreme Lord without the transparent via medium of the spiritual master. "Transparent" means that the via medium must be free of contamination. If it is transparent, one can see through it.

not really, at least the way it is usually explained. we should be able to see Krsna through his actions and speach. when you see glasses, you see glasses. when you look THROUGH the glasses, you see what is on the other side. if the glasses are opaque, you still see glasses, and glasses only.

Interesting. So, to use the eyeglasses analogy, a medium may be transparent, but may distort the light passing through it to make it more accessible to the eye?

I don't see too many transparent media gurus. So many are spinning their own versions of the siddhanta, with their favorite catch phrases and concepts.
Hridayananda speaks of psychological atheism as opposed to philosophical atheism. This is manufacturing, since there is nothing in Prabhupada's purports that is analogous to it. He has other jnani concepts that don't map to Prabhupada's teachings.
The guru needs to simply transmit what his guru transmitted like a succession of transparencies from Krsna. No concoctions , special treatise, white papers and all the trappings of jnani speculation that I have read so often.

SB 4.8.44p:
It is here recommended to Dhruva Mahäräja that he meditate on the supreme guru, or supreme spiritual master. The supreme spiritual master is Krsna, who is therefore known as caitya-guru. This refers to the Supersoul, who is sitting in everyone’s heart. He helps from within as stated in Bhagavad-gita, and He sends the spiritual master, who helps from without. The spiritual master is the external manifestation of the caitya-guru, or the spiritual master sitting in everyone’s heart.

Bhaktivinoda, Bhaktisiddhanta, and Prabhupada each developed different ways of presenting the philosophy, with their own special concepts and even different ways of understanding the siddhanta. Do you find the concept of "BACK to Godhead" in our sampradaya prior to Srila Prabhupada for example? I doubt it. Yet I see great transparency in him, perhaps as much as can ever be expected.

Yes, that is the way I see it. Krsna teaches us from within and from without. Guru is the best external manifestation of Adi-Guru. An interactive media presentation of the Supersoul.

Grammar quibble...

"Media" is plural. "Medium" is singular. Ought to be "Guru as a transparent medium".

Does that definition make sense in this context?

It seems like definition number 3 of "medium" is what applies here.

From:
http://www.thefreedictionary.com/medium.

All I know is that Srila Prabhupada used the terminology "transparent via media".

I don't remember ever hearing or reading him say "transparent medium".
He most always said "transparent via media".

I don't know if he is the origin of that usage or if his Gurudeva used it before him.

I've read the grandfather acaryas and they don't multiply concepts and 'different ways of understanding'. That is the jnani marga. The path of ascent. They have their own style - not their own philosohpical version. They also emphasize different aspects of the siddhanta.
I was amazed how similar the content was between these acaryas, in spite of their different writing styles. And yes Bhaktivinode Thakur does mention going back home (though not in those words) in his Caitanya siksamrita.

The thing that concerns me is the difference between the way an impersonalist hears the term transparent via medium to God and the way the Vaisnava hears the same phrase.

The impersonalist takes transparent to mean the person of guru has disappeared into the Brahman and so no more of his personality is in the way.

The Vaisnava, especially the followers of Mahaprabhu, see the real person of Guru has been awakened to his natural position as being both one and different from the Supreme Brahman.

He is not only still there with Krsna (and as Krsna) but he is there in his realized unchanging state. We accept the liberated Vaisnava Acarya as the Supreme Personality of Servitor Godhead.

So when we look at such a guru we see him and Krsna simultaneously. Transparent just means free from the darkness of ahankara. We have to be careful that in our mind we are not blanking guru out to get to God.

I don't understand, are we not suppose to worship Guru as God?

I disagree that transparency just relates to the lack of ahankara. That is not what Prabhupada intended. He indicates that we can see Krsna thanks to our guru being a transparent window into the spiritual world.

If you dont see Krsna speaking and acting through a particular person you should not accept them as your guru. Plain and simple.

I think that in the way Srila Prabhupada said "transparent via media" it means more than just transparent but also "via media" which is like the go-between, intermediary between Krishna and the jivas.
Transparent via media......?
A via media that is transparent.
But, the guru is not just a window to the spiritual world.
He is also the intermediary between Krishna and the conditioned souls.

Yes but the point is he must be factually situated in the liberated position which means free from ahankara (falsely identity) AND established in his natural position as Servitor Godhead.

It is not enough to be free from ahankara alone. That is seen in impersonalism as well. One must be liberated and fixed in relationship to Krsna then he is fit to be worshipped as God.

To accept a kanishta adhikari or even a second class person as guru and then worship him as God is not an intelligent thing to do. Prabhupada teaches one should accept a first class devotee as guru.