When a lonely political prisoner in Chile or Iran thinks of the Red Cross, blood-mobiles probably do not come to mind. For him, the famous emblem means the only link he has with his family outside prison walls.

Although the International Committee of the Red Cross is better known for its aid to prisoners of war, the small, tightly run all-Swiss operation headquartered here has quietly become the one human rights group actually penetrating jails throughout the world.

Last year alone, it visited some 14,000 political prisoners in 22 countries.

Delegates go prepared with a check list of 400 questions on conditions ranging from prison food to torture and family visits.

They normally interview the prisoners without witnesses. Sometimes they provide assistance to the prisoners' families, which often are ostracized.

While such high-profile and well funded groups as the U.N. Human Rights Commission and Amnesty International splash in public their vivid condemnations of rights violations, the publicity shy Red Cross, with a limited budget and a 340 "dash" member staff, has probably managed to do more - in terms of improving prison conditions - than any other similar organization.

This is partly because the Red Cross, carrying on a wartime tradition of avoiding politics, confines itself to seeking humane prison conditions and stays clear of the more sensitive questions of legal justice that are central to groups like Amnesty.

With Amnesty publicly pressuring governments to release detainees on grounds of justice and the International Red Cross privately improving prisoner's conditions on humanitarian grounds, the two groups play different but complementary roles.

Although some have criticized the International Red Cross for cooperating with dictators when it ignores the reasons for detentions, the Red Cross maintains that it must remain neutral to do any good for individuals.

Some critics, according to David Fosythe, who has studied Red Cross activity, argue that "the ICRC's presence works primarily to the advantage of the detaining authority in that what should not even exist is approved by the ICRC as humanitarian."

Furthermore, the critics say, dictators take advantage of the public relations value of the Red Cross visits to dampen international public opinion against them.

Although aware that improving a government's image may be a side effect of its work, the International Red Cross maintains that public relations is one of the main reasons a government will allow the organization to visit - and thus physically help - prisoners.

The Red Cross's secretiveness is also a key factor. The intricately detailed reports on prison conditions go only to the government concerned. The committee, through its small public relations office here, usually provides only brief mention of places of detention visited and no clues as to the actual conditions found.

Governments would cancel the visits if the International Red Cross broke its promise of confidentiality.

"Discretion is part of the game," said a Red Cross official, adding that it is, in effect, the ticket for admission to the jails.

But the outspoken approach of the Carter administration on human rights has helped the committees work, according to many Red Cross delegates and other observers here, by providing the kind of international public pressure that the Red Cross cannot initiate but can take advantage of in persuading governments to accept their services.

There are now plans within the U.S. administration to roughly double the U.S. Contribution of $500,000 to the budget over the next two years.

As more governments accept the idea of International Red Cross prison visits, demands for its services are outstripping the organization's regular budget of about $12 million, which is augmented by funds raised by special appeals during crises.

Despite gaining governmental permission, the International Red Cross is not able to visit prisoners in a number of African and Latin American countries because of a lack of funds and personnel. The offices in Togo and Venezuela have been closed.

While outside pressure and public images are important, when delegates approach governments they play on other motives as well. The technique is to convince a government that accepting help for its imprisoned opponents is really in its own interests.

Delegates argue, for example, that they can help "well-intentioned" leaders make sure their humanitarian directives are actually being carried out by wardens and other subordinates.

Sometimes government ministers genuinely are surprised by what delegates find. In one Middle Eastern country, a delegate placed on the prime minister's desk a map of a prison room drawn to scale and then piled up paper dolls on it to dramatize how horribly overcrowded sleeping conditions were. The shocked prime minister reformed the sleeping quarters.

A classic example of the Red Cross using outside pressure, according to an expert on prisoners, occurred in Greece during the rule of the colonels in the late 1960s. Faced with a hostile American and European press and the prospect of being expelled from the Council of Europe over charges of violating the European Convention on Human Rights, the military regime signed in 1969 an agreement for the committee to visit all places of political detention.

The accord, which included the unusual right to visit interrogation centers, greatly strengthened the Red Cross' ability to protect Greek prisoners.During the next year, allegations of torture virtually ceased. But in 1970, after foreign pressure had eased, the Greek colonels refused to renew the agreement. The cutoff, shrugs a Red Cross lawyer who was involved, "got only five lines in Le Monde."

According to diplomatic sources here, the shah of Iran, who last year began allowing International Red Cross visits, has used the reports to check on subordinates and make changes in prison conditions.

When trying to enter countries, the committee also points out to governments that the visits do not affect the legal status of the prisoners. In fact, while the Red Cross aid to war victims is covered under the Geneva Conventions, the organization has no legal basis for protecting political prisoners.

But this lack of legal grounds limits room to maneuver. When governments try to use the Red Cross to shield themselves from criticism, one critic warns, discretion can be "comfortable to both the controlling authority and to the Red Cross."

Some governments have found, though, that even Red Cross discretion has its limits. When the government controlled media in Uruguay quoted the Red Cross as saying "the prisons in Uruguay are models," it shot back a press release stating that while it never comments on detention conditions, it was "unable to make an objective and complete assessment of conditions" because of restrictions imposed by authorities, such as requiring witnesses for interviews.

While it is difficult to assess what impact the Red Cross has had in helping political prisoners, given the scape of the problems and the secrecy surrounding its work, at least it is clear that the prisoners themselves - sometimes after initial suspicion - want its services continued.

A visit from relatives, an extra blanket, an exercise area arranged by a Red Cross delegate all help.

Perhaps a prisoner explained it best when he confided to a delegate, "Never forget it's not so much the good you bring, but the bad you prevent."